It looks like QThreadPool has some thread synchronization defect in
expiration mechanism.

I am using QThreadPool for running some tasks in a separate thread. I have
faced the following problem. Sometimes the task (QRunnable descendant) does
not start despite of the pool is idle, i.e. activeThreadCount() is 0. The
bug is reproduced when start() is called simultaneously with the moment of
thread life's expiration due to 30-second timeout. I see that starting of
another task causes thread pool to start the frozen one as well.