Remember when I said, "My guess is that it's something stupid?" :-D
The problem turned out to be the ID number assigned to the "conv"
category of built-in verbs. I had been using 1026, but when I rolled
in the new sqlite and mysql verbs from trunk they used 1026 and 1027.
The overlapping verb-category ID numbers were causing the verb
initializations to fail when the app started up. This was triggering
an assertion failure, which (unfortunately) is handled in a threaded
fashion... but the failure was happening before the main thread had
been initialized. In the end, the crash was happening in the code
which attempts to deal with assertion failures.
Or that's how it looked to me, anyway. I can reproduce it at will, but
the problem is fixed.
Our handling of assertion failures before the main thread is
initialized may be a bug. ::shrug:: Hard to give it any priority,
though, as you only trigger it when you've broken something else.