In the documentation for Python 2.7.3 (looking at 14 Dec version) under section 16.2.2, entitled "Lock Objects", in the second paragraph is the sentence: If an attempt is made to release an unlocked lock, a RuntimeError will be raised. This is not true. As it says farther down under "Lock.release()": When invoked on an unlocked lock, a ThreadError is raised. So it may be a "runtime error" as opposed to a syntax error, but it isn't a RuntimeError. It's a threading.ThreadError, which doesn't inherit from RuntimeError or even StandardError, but from Exception. Feel free to contact me if this is unclear.