Question about semaphores and threads

[Marc CHALAND]

Hi,

Sometimes, I get the following message :
semaphore/lib/src/semaphore.c:167:__enqueue_thread():
  Error : l4semahore: failed to get priority of thread 17.08: invalid
argument (-3)

Into sources, the comment says that thread has not been created by
l4thread or is dead.
But all threads are created by l4thread and no one is explicitly
killed by another thread. Is there a mechanism which kills threads ?
Is it possible a lock is requested as thread is not completely created
? Anybody has got an idea on what happens ?

Regards
Marc