I'll have a look at it, it seems like there's some inconsistency in the scheduler...

do you happen to have an example of a program that causes such a crash - or any guess, which events or function seem to cause this problem more often ?

edit :
do you use two threads in your program ? because i think that this problem could arise if the "engine"-thread (that iterates over the "yield return" functiosn) exists AND ANOTHER thread that somehow also tries to interact with that thread or the engine !