Found the bug- apparently there was an issue with a loop in the unit's opening sequence (before it reports INBUILDSTANCE) that used to operate all right, but now needed a sleep put in, to avoid a lock.
Why was COB changed, and what was changed this time? The Changelog is silent about it, and with the mess with Git changeover, etc., I can't see every commit right now. I'll subscribe to the RSS feed, this is really not good, not even knowing where to check.
Gonna check everything now, I guess I don't get to sleep tonight

I'd strongly suggest that other people making games check every single COB for similar lock conditions.
2. Spring apparently no longer considers treetypes to be Features, so my first big World Builder demo map is trashed, until I go and replace the trees.
3. I've watched aircraft fly through hills, and they don't seem to be trying to obey their CruiseAlt during combat now, which means that they fly right through World Builder cities during firefights, because they have collide off, and refuse to try and stay near CruiseAlt.
4. TASClient just flat-out crashed on one map, no warning, nothing, just blip, gone.
5. Neither AAI nor KAIK can fail gracefully, both crash. Is it too much to ask for some better exception handling on them, so that they don't take Spring down, and give me something useful in the way of an error?
I think KAIK is crashing mainly because the new P.U.R.E. maps are metal, dunno for sure yet.
6. The sound volume control in SpringSettings is still screwed up. You can't turn down unit replies, without turning down the main sounds as well. Probably still referring to the wrong variable.
[EDIT]The sound volume vs. sound volume for OGG streams is very, very quiet now, that's another big problem. I'll just fix my Widget, set the volumes lower.[/EDIT]
[EDIT2]No, the problem is that OGG is no longer obeying the parameter that sets the volume.[/EDIT]
[EDIT3]Confirmed, Spring.SetSoundStreamVolume() doesn't work, either, it appears it's just broken, period.[/EDIT]
7. I see little holes in the map, if shadows are off:

Dunno what's up with that, looks like a really weird shader bug. I saw this before 0.77b5, tbh I just forgot to document it, so this isn't "new", but it's still a bug.