[1.20.1] Pondie causing client-side disconnect and client can't log back in

[XxInvictus]

This has been happening in a repeatable fashion. Upon going to a certain location the client will be flooded with these error's and get hard-disconnected. They also will not be able to login while in that location as they will join, the world will load, then they will get flooded with the same error, rinse and repeat. The only way we have been able to recover a player is by quickly teleporting them in the seconds between the join/disconnect.

I have tried to /kill all of these entities from the area but as soon as the player spawns in, they get spawned again and it repeats.

MC Version: 1.20.1
Fish of Thieves version: 3.2.1

[22:40:13] [Render thread/ERROR] [minecraft/BlockableEventLoop]: Error executing task on Client
java.lang.IllegalStateException: Invalid entity data item type for field 21 on entity Pondie['Pondie'/2103, l='ClientLevel', x=329.91, y=49.29, z=1622.31]: old=com.stevekung.fishofthieves.entity.variant.PondieVariant@54d398e1(class com.stevekung.fishofthieves.entity.variant.PondieVariant), new=com.stevekung.fishofthieves.entity.variant.SplashtailVariant@e792bc2(class com.stevekung.fishofthieves.entity.variant.SplashtailVariant)
	at net.minecraft.network.syncher.SynchedEntityData.m_135375_(SynchedEntityData.java:205) ~[client-1.20.1-20230612.114412-srg.jar%231212!/:?] {re:mixin,pl:accesstransformer:B,pl:connector_pre_launch:A,re:classloading,pl:accesstransformer:B,pl:mixin:APP:modernfix-common.mixins.json:core.SynchedEntityDataMixin from mod modernfix,pl:mixin:APP:lithium.mixins.json:entity.data_tracker.use_arrays.DataTrackerMixin from mod radium,pl:mixin:APP:alltheleaks.mixins.json:main.SynchedEntityDataMixin from mod alltheleaks,pl:mixin:APP:lithium.mixins.json:entity.data_tracker.no_locks.DataTrackerMixin from mod radium,pl:mixin:APP:forge-badoptimizations.mixins.json:entitydata.MixinSynchedData from mod (unknown),pl:mixin:A,pl:connector_pre_launch:A}
	at net.minecraft.network.syncher.SynchedEntityData.m_135356_(SynchedEntityData.java:192) ~[client-1.20.1-20230612.114412-srg.jar%231212!/:?] {re:mixin,pl:accesstransformer:B,pl:connector_pre_launch:A,re:classloading,pl:accesstransformer:B,pl:mixin:APP:modernfix-common.mixins.json:core.SynchedEntityDataMixin from mod modernfix,pl:mixin:APP:lithium.mixins.json:entity.data_tracker.use_arrays.DataTrackerMixin from mod radium,pl:mixin:APP:alltheleaks.mixins.json:main.SynchedEntityDataMixin from mod alltheleaks,pl:mixin:APP:lithium.mixins.json:entity.data_tracker.no_locks.DataTrackerMixin from mod radium,pl:mixin:APP:forge-badoptimizations.mixins.json:entitydata.MixinSynchedData from mod (unknown),pl:mixin:A,pl:connector_pre_launch:A}
	at net.minecraft.client.multiplayer.ClientPacketListener.m_6455_(ClientPacketListener.java:502) ~[client-1.20.1-20230612.114412-srg.jar%231212!/:?] {re:mixin,pl:accesstransformer:B,pl:connector_pre_launch:A,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:APP:fabric-message-api-v1.client.mixins.json:ClientPlayNetworkHandlerMixin from mod fabric_message_api_v1,pl:mixin:APP:fabric-networking-api-v1.client.mixins.json:ClientPlayNetworkHandlerMixin from mod fabric_networking_api_v1,pl:mixin:APP:forgematica.mixins.json:MixinClientPlayNetworkHandler from mod forgematica,pl:mixin:APP:pickupnotifier.common.mixins.json:client.ClientPacketListenerMixin from mod pickupnotifier,pl:mixin:APP:mafglib.mixins.json:MixinClientPlayNetworkHandler from mod mafglib,pl:mixin:APP:almostunified-common.mixins.json:runtime.ClientPacketListenerMixin from mod almostunified,pl:mixin:APP:hybrid-aquatic.forge.mixins.json:client.ClientPlayNetworkHandlerMixin from mod hybrid_aquatic,pl:mixin:APP:mixins.oculus.json:MixinClientPacketListener from mod oculus,pl:mixin:APP:enhancedvisuals.mixins.json:ClientPacketListenerMixin from mod enhancedvisuals,pl:mixin:APP:xaerolib.mixins.json:MixinClientPacketListener from mod xaerolib,pl:mixin:APP:fancymenu.mixins.json:client.MixinClientPacketListener from mod fancymenu,pl:mixin:APP:graveyard-common.mixins.json:ClientPlayNetworkHandlerMixin from mod graveyard,pl:mixin:APP:pehkui.mixins.json:client.compat117plus.ClientPlayNetworkHandlerMixin from mod pehkui,pl:mixin:APP:bettercombat.mixins.json:client.ClientPlayNetworkHandlerMixin from mod bettercombat,pl:mixin:APP:mixins/common/nochatreports.mixins.json:client.MixinClientPacketListener from mod nochatreports,pl:mixin:APP:mixins/forge/nochatreports-forge.mixins.json:client.MixinClientPacketListener from mod (unknown),pl:mixin:APP:xaerohud.mixins.json:MixinClientPlayNetworkHandler from mod xaerominimap,pl:mixin:APP:mixins.universaloptimization.json:FMC from mod universaloptimization,pl:mixin:APP:aether.mixins.json:client.ClientPacketListenerMixin from mod aether,pl:mixin:APP:xaeroworldmap.mixins.json:MixinClientPlayNetworkHandler from mod xaeroworldmap,pl:mixin:APP:xaeroworldmap.forge.mixins.json:MixinForgeClientPacketListener from mod xaeroworldmap,pl:mixin:APP:fabric-data-attachment-api-v1.client.mixins.json:ClientPlayNetworkHandlerMixin from mod fabric_data_attachment_api_v1,pl:mixin:APP:railways.mixins.json:client.ClientPacketListenerMixin from mod railways,pl:mixin:APP:fabric-lifecycle-events-v1.client.mixins.json:ClientPlayNetworkHandlerMixin from mod fabric_lifecycle_events_v1,pl:mixin:APP:fastsuite.mixins.json:client.ClientPacketListenerMixin from mod fastsuite,pl:mixin:APP:mixins.cofhcore.json:ClientPacketListenerMixin from mod cofh_core,pl:mixin:APP:endergetic.mixins.json:client.ClientPacketListenerMixin from mod endergetic,pl:mixin:APP:moonlight-common.mixins.json:ClientPacketListenerMixin from mod moonlight,pl:mixin:APP:iceberg.mixins.json:ClientPacketListenerMixin from mod iceberg,pl:mixin:APP:presencefootsteps.mixin.json:MClientPlayNetworkHandler from mod presencefootsteps,pl:mixin:APP:sound_physics_remastered.mixins.json:ClientPacketListenerMixin from mod (unknown),pl:mixin:APP:embeddium.mixins.json:core.world.map.ClientPlayNetworkHandlerMixin from mod embeddium,pl:mixin:APP:the_bumblezone-common.mixins.json:client.ClientPacketListenerMixin from mod the_bumblezone,pl:mixin:A,pl:connector_pre_launch:A,pl:runtimedistcleaner:A}
	at net.minecraft.network.protocol.game.ClientboundSetEntityDataPacket.m_5797_(ClientboundSetEntityDataPacket.java:46) ~[client-1.20.1-20230612.114412-srg.jar%231212!/:?] {re:classloading,pl:connector_pre_launch:A}
	at net.minecraft.network.protocol.game.ClientboundSetEntityDataPacket.m_5797_(ClientboundSetEntityDataPacket.java:10) ~[client-1.20.1-20230612.114412-srg.jar%231212!/:?] {re:classloading,pl:connector_pre_launch:A}
	at net.minecraft.client.multiplayer.ClientPacketListener.m_264308_(ClientPacketListener.java:2305) ~[client-1.20.1-20230612.114412-srg.jar%231212!/:?] {re:mixin,pl:accesstransformer:B,pl:connector_pre_launch:A,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:APP:fabric-message-api-v1.client.mixins.json:ClientPlayNetworkHandlerMixin from mod fabric_message_api_v1,pl:mixin:APP:fabric-networking-api-v1.client.mixins.json:ClientPlayNetworkHandlerMixin from mod fabric_networking_api_v1,pl:mixin:APP:forgematica.mixins.json:MixinClientPlayNetworkHandler from mod forgematica,pl:mixin:APP:pickupnotifier.common.mixins.json:client.ClientPacketListenerMixin from mod pickupnotifier,pl:mixin:APP:mafglib.mixins.json:MixinClientPlayNetworkHandler from mod mafglib,pl:mixin:APP:almostunified-common.mixins.json:runtime.ClientPacketListenerMixin from mod almostunified,pl:mixin:APP:hybrid-aquatic.forge.mixins.json:client.ClientPlayNetworkHandlerMixin from mod hybrid_aquatic,pl:mixin:APP:mixins.oculus.json:MixinClientPacketListener from mod oculus,pl:mixin:APP:enhancedvisuals.mixins.json:ClientPacketListenerMixin from mod enhancedvisuals,pl:mixin:APP:xaerolib.mixins.json:MixinClientPacketListener from mod xaerolib,pl:mixin:APP:fancymenu.mixins.json:client.MixinClientPacketListener from mod fancymenu,pl:mixin:APP:graveyard-common.mixins.json:ClientPlayNetworkHandlerMixin from mod graveyard,pl:mixin:APP:pehkui.mixins.json:client.compat117plus.ClientPlayNetworkHandlerMixin from mod pehkui,pl:mixin:APP:bettercombat.mixins.json:client.ClientPlayNetworkHandlerMixin from mod bettercombat,pl:mixin:APP:mixins/common/nochatreports.mixins.json:client.MixinClientPacketListener from mod nochatreports,pl:mixin:APP:mixins/forge/nochatreports-forge.mixins.json:client.MixinClientPacketListener from mod (unknown),pl:mixin:APP:xaerohud.mixins.json:MixinClientPlayNetworkHandler from mod xaerominimap,pl:mixin:APP:mixins.universaloptimization.json:FMC from mod universaloptimization,pl:mixin:APP:aether.mixins.json:client.ClientPacketListenerMixin from mod aether,pl:mixin:APP:xaeroworldmap.mixins.json:MixinClientPlayNetworkHandler from mod xaeroworldmap,pl:mixin:APP:xaeroworldmap.forge.mixins.json:MixinForgeClientPacketListener from mod xaeroworldmap,pl:mixin:APP:fabric-data-attachment-api-v1.client.mixins.json:ClientPlayNetworkHandlerMixin from mod fabric_data_attachment_api_v1,pl:mixin:APP:railways.mixins.json:client.ClientPacketListenerMixin from mod railways,pl:mixin:APP:fabric-lifecycle-events-v1.client.mixins.json:ClientPlayNetworkHandlerMixin from mod fabric_lifecycle_events_v1,pl:mixin:APP:fastsuite.mixins.json:client.ClientPacketListenerMixin from mod fastsuite,pl:mixin:APP:mixins.cofhcore.json:ClientPacketListenerMixin from mod cofh_core,pl:mixin:APP:endergetic.mixins.json:client.ClientPacketListenerMixin from mod endergetic,pl:mixin:APP:moonlight-common.mixins.json:ClientPacketListenerMixin from mod moonlight,pl:mixin:APP:iceberg.mixins.json:ClientPacketListenerMixin from mod iceberg,pl:mixin:APP:presencefootsteps.mixin.json:MClientPlayNetworkHandler from mod presencefootsteps,pl:mixin:APP:sound_physics_remastered.mixins.json:ClientPacketListenerMixin from mod (unknown),pl:mixin:APP:embeddium.mixins.json:core.world.map.ClientPlayNetworkHandlerMixin from mod embeddium,pl:mixin:APP:the_bumblezone-common.mixins.json:client.ClientPacketListenerMixin from mod the_bumblezone,pl:mixin:A,pl:connector_pre_launch:A,pl:runtimedistcleaner:A}
	at net.minecraft.network.protocol.game.ClientboundBundlePacket.m_5797_(ClientboundBundlePacket.java:13) ~[client-1.20.1-20230612.114412-srg.jar%231212!/:?] {re:classloading,pl:connector_pre_launch:A}
	at net.minecraft.network.protocol.game.ClientboundBundlePacket.m_5797_(ClientboundBundlePacket.java:6) ~[client-1.20.1-20230612.114412-srg.jar%231212!/:?] {re:classloading,pl:connector_pre_launch:A}
	at net.minecraft.network.protocol.PacketUtils.m_263899_(PacketUtils.java:22) ~[client-1.20.1-20230612.114412-srg.jar%231212!/:?] {re:mixin,pl:connector_pre_launch:A,re:classloading,pl:connector_pre_launch:A}
	at net.minecraft.util.thread.BlockableEventLoop.m_6367_(BlockableEventLoop.java:156) ~[client-1.20.1-20230612.114412-srg.jar%231212!/:?] {re:mixin,pl:accesstransformer:B,pl:connector_pre_launch:A,re:computing_frames,pl:accesstransformer:B,pl:connector_pre_launch:A,re:classloading,pl:accesstransformer:B,pl:connector_pre_launch:A}
	at net.minecraft.util.thread.ReentrantBlockableEventLoop.m_6367_(ReentrantBlockableEventLoop.java:23) ~[client-1.20.1-20230612.114412-srg.jar%231212!/:?] {re:mixin,pl:connector_pre_launch:A,re:computing_frames,pl:connector_pre_launch:A,re:classloading,pl:connector_pre_launch:A}
	at net.minecraft.util.thread.BlockableEventLoop.m_7245_(BlockableEventLoop.java:130) ~[client-1.20.1-20230612.114412-srg.jar%231212!/:?] {re:mixin,pl:accesstransformer:B,pl:connector_pre_launch:A,re:computing_frames,pl:accesstransformer:B,pl:connector_pre_launch:A,re:classloading,pl:accesstransformer:B,pl:connector_pre_launch:A}
	at net.minecraft.util.thread.BlockableEventLoop.m_18699_(BlockableEventLoop.java:115) ~[client-1.20.1-20230612.114412-srg.jar%231212!/:?] {re:mixin,pl:accesstransformer:B,pl:connector_pre_launch:A,re:computing_frames,pl:accesstransformer:B,pl:connector_pre_launch:A,re:classloading,pl:accesstransformer:B,pl:connector_pre_launch:A}
	at net.minecraft.client.Minecraft.m_91383_(Minecraft.java:1106) ~[client-1.20.1-20230612.114412-srg.jar%231212!/:?] {re:mixin,pl:accesstransformer:B,pl:connector_pre_launch:A,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:A,pl:connector_pre_launch:A,pl:runtimedistcleaner:A}
	at net.minecraft.client.Minecraft.m_91374_(Minecraft.java:718) ~[client-1.20.1-20230612.114412-srg.jar%231212!/:?] {re:mixin,pl:accesstransformer:B,pl:connector_pre_launch:A,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:A,pl:connector_pre_launch:A,pl:runtimedistcleaner:A}
	at net.minecraft.client.main.Main.main(Main.java:218) ~[minecraft-1.20.1-client.jar:?] {re:mixin,pl:connector_pre_launch:A,pl:runtimedistcleaner:A,re:classloading,pl:mixin:APP:cubes-without-borders.mixins.json:MainMixin from mod cubes_without_borders,pl:mixin:APP:mixins.emibridge.json:MainEntrypointMixin from mod connectorextras_emi_bridge,pl:mixin:A,pl:connector_pre_launch:A,pl:runtimedistcleaner:A}
	at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:?] {}
	at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[?:?] {}
	at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?] {}
	at java.lang.reflect.Method.invoke(Method.java:568) ~[?:?] {re:mixin}
	at net.minecraftforge.fml.loading.targets.CommonLaunchHandler.runTarget(CommonLaunchHandler.java:111) ~[fmlloader-1.20.1-47.4.10.jar:?] {}
	at net.minecraftforge.fml.loading.targets.CommonLaunchHandler.clientService(CommonLaunchHandler.java:99) ~[fmlloader-1.20.1-47.4.10.jar:?] {}
	at net.minecraftforge.fml.loading.targets.CommonClientLaunchHandler.lambda$makeService$0(CommonClientLaunchHandler.java:25) ~[fmlloader-1.20.1-47.4.10.jar:?] {}
	at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:30) ~[modlauncher-10.0.9.jar:?] {}
	at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:53) ~[modlauncher-10.0.9.jar:?] {}
	at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:71) ~[modlauncher-10.0.9.jar:?] {}
	at cpw.mods.modlauncher.Launcher.run(Launcher.java:108) ~[modlauncher-10.0.9.jar:?] {}
	at cpw.mods.modlauncher.Launcher.main(Launcher.java:78) ~[modlauncher-10.0.9.jar:?] {}
	at cpw.mods.modlauncher.BootstrapLaunchConsumer.accept(BootstrapLaunchConsumer.java:26) ~[modlauncher-10.0.9.jar:?] {}
	at cpw.mods.modlauncher.BootstrapLaunchConsumer.accept(BootstrapLaunchConsumer.java:23) ~[modlauncher-10.0.9.jar:?] {}
	at cpw.mods.bootstraplauncher.BootstrapLauncher.main(BootstrapLauncher.java:141) ~[bootstraplauncher-1.1.2.jar:?] {}
	at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:?] {}
	at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[?:?] {}
	at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?] {}
	at java.lang.reflect.Method.invoke(Method.java:568) ~[?:?] {re:mixin}
	at io.github.zekerzhayard.forgewrapper.installer.Main.main(Main.java:67) ~[?:?] {}
	at org.prismlauncher.launcher.impl.StandardLauncher.launch(StandardLauncher.java:105) ~[?:?] {}
	at org.prismlauncher.EntryPoint.listen(EntryPoint.java:129) ~[?:?] {}
	at org.prismlauncher.EntryPoint.main(EntryPoint.java:70) ~[?:?] {}

[SteveKunG]

Could you upload your mod pack? This seems look like mod version mismatch between client and server, but don't get me wrong I'm not entirely sure until I got modpack to test with.

[XxInvictus]

It is a custom modpack with ~530 mods so not sure it would be practical to run locally, I can definitely upload a copy just a pre-warning 😆
I can even provide a world-backup and send the exact problem coordinates if you feel like the pain of a 7-8GB world backup 😆

If it helps at all here is the log line showing the Mod Optimizer output for server and client where FoT is the same on both sides.
Server:

[06Jan2026 17:53:48.230] [main/INFO] [Mods Optimizer/]: | fishofthieves                      | 3.2.1                  | FORGE    | BOTH              | 2025-12-30 13:00:38 |

Client:

[06Jan2026 22:29:51.084] [main/INFO] [Mods Optimizer/]: | fishofthieves                      | 3.2.1                  | FORGE    | BOTH              | 2025-12-30 23:28:32 |

[XxInvictus]

Just to clarify, I am not 100% convinced this is the cause of the crash, it may be a side-effect rather than a cause. I am still doing some investigation/testing but this is the only error being shown to the client.

[SteveKunG]

I don't mind if you send a 10GB of your modpack and map haha, I just want to know what's really happened with the mod. Especially big mod pack.