_socket_semaphore.acquire() is called in Pool.get_socket() before we attempt to pop a socket from the set of sockets or create a new socket. Unfortunately, if __check fails and/or we can't create a new socket we don't call _socket_semaphore.release(). This causes all application threads to hang once the semaphore's value reaches 0.

https://groups.google.com/forum/#!topic/mongodb-user/l0PY8zsjZ3c