MOUSEMOTION prevents MOUSEBUTTONUP event
If the mouse is in motion when you release the button 1, a MOUSEBUTTONUP event is NOT added to the event queue in Windows pygame 1.9.1. I do not know if this applies to buttons 2 or 3, any MOUSEBUTTONDOWN, or later versions of pygame. There is a work around, testing for either a MOUSEBUTTONUP or a MOUSEMOTION BUTTONS status change. If not fixable or not worth fixing, the problem and work around might be worth noting in the documentation.