From the manpage of pthread_cond_wait():

{block}
When using condition variables there is always a Boolean predicate involving shared variables associated with each condition wait that is true if the thread should proceed. Spurious wakeups from the pthread_cond_timedwait() or pthread_cond_wait() functions may occur. Since the return from pthread_cond_timedwait() or pthread_cond_wait() does not imply anything about the value of this predicate, the predicate should be re-evaluated upon such return.{block}

Basically, pthread_cond_wait can return whenever it feels like! Our code in the mock server (and maybe elsewhere) doesn't deal with this case, and it should.