EDIT: Well, after systematically deleting every single asset from the level and re-cooking until something changed, I believe I have it narrowed down to Procedural Mesh Components misbehaving.
I’ll play around with this and hopefully have a set of repro steps up… or I could just upload the asset in question.
EDIT 2: Create a Procedural Mesh Component at runtime. Set it to generate collision. Set the asset to have CCD.
This is what generated a cook failure for ME.
But the thing is, other users have mentioned that this has happened with stuff like trigger volumes instead, that never threw CCD errors until 4.12 (much as my Proc Mesh Comps cooked fine in 4.11)… the issue is that something LARGER is going on with the way the CCD “warning” flag is actually being interpreted as an error flag. Even if you can repro it through my steps, I feel like the actual BUG is something a bit more on the back-end with how that “warning” is being handled.