ICEfaces application bugs exposed a thread leak in the Thread Blocking environment. If an exception is thrown while trying to send a response to a request the Servlet Container supplied thread got blocked and never got unblocked. The Thread Blocking environment relies on semaphore logic to block and unblock the Servlet Container supplied thread. This thread tries to acquire the semaphore if no response is available causing this thread to block. When a response becomes available another thread releases the semaphore in order to unblock the Servlet Container supplied thread. If an exception occurred during this process the semaphore was not released.

Basically, an exception during Request.respondWith(...) causes ThreadBlockingAdaptingServlet's semaphore to not be released:

    public void respondWith(ResponseHandler handler) throws Exception {
        super.respondWith(handler);
        if (semaphore == null) {
            blockResponse = false;
        } else {
            semaphore.release();
        }
    }