Two threads, started at the same level (ie, both spawned by main) have the problem that when one thread exits, it freezes the other thread. The problem is shown in the attached code - it might be new in versions 25-26 of the NBC compiler, i havent tried it on older versions.
The code has two threads that just loop printing to the screen. If the jmp is commented in the first thread, it should exit after one pass - this also freezes the second thread, but does not stop the program.
(I dont know if this is a bug, or improperly used threads)
(and thanks to Andrew for the code and showing me the bug)