This was added in VBSP++, so it's likely to be becoming more common (especially in TF2 maps). If it's anything like the PR at ValveSoftware/source-sdk-2013#1323, it'll be caused by a combination of CONTENTS_DETAIL + CONTENTS_OPAQUE; some tests may be necessary to verify that, or you could try asking @ficool2 and hope he answers.
There should be a checkbox to treat them as regular func_detail instead, since vanilla compilers don't support it.
This was added in VBSP++, so it's likely to be becoming more common (especially in TF2 maps). If it's anything like the PR at ValveSoftware/source-sdk-2013#1323, it'll be caused by a combination of CONTENTS_DETAIL + CONTENTS_OPAQUE; some tests may be necessary to verify that, or you could try asking @ficool2 and hope he answers.
There should be a checkbox to treat them as regular func_detail instead, since vanilla compilers don't support it.