In normal operation with pthreads when PyThread_release_lock is called and it calls PyThread_acquire_lock in the "sanity check" errno will be set to 11 and left that way.  It seems like this should not happen so thread switches don't arbitrarily and w/o much reason alter the program state.

I ran into this working on a threaded debugger where a lock is used internally.  The debugger I'm writing now saves/restores errno in its tracer but it seems like there is potential for this bug all over the place.