diff --git a/pages/debugging.md b/pages/debugging.md
index 4da6cfb..f133936 100644
--- a/pages/debugging.md
+++ b/pages/debugging.md
@@ -26,6 +26,17 @@ consoles and link budgets, with the Inspector for whichever node is selected.
 | no route | nothing relayed it | regions, `flood_max`, loop detection |
 | dropped by region | the node does not hold the scope | the region map, or the scope being sent on |
 
+A collision says where it landed. "Decoded its header, then N symbol(s) were
+destroyed" is a frame whose payload failed its CRC; "locked on, then N of its 8
+header symbols were destroyed" is a header the receiver could not read. The
+receiving node's firmware is told the same thing its radio would report: a CRC
+error, with the damaged bytes, or a header error and nothing more. A frame the
+receiver could detect and never locked onto, because something stronger
+swamped it while its preamble was still to be found, raises nothing, as on the
+radio: on a native node the preamble flag stays set until the firmware clears
+it, and a receiver re-armed part-way through a frame loses that frame. Only
+native firmware builds that support it are told; emulated boards are not yet.
+
 "It did not arrive" and "it arrived 3 dB under the floor" lead to completely
 different fixes, which is why the cause is recorded rather than a boolean.