Hi,
FWIW I am unable to reproduce on Win 7 Pro, Py 3.5.1, PyGame 1.9.2a0 (to print the latter, `print(pygame.ver)`).
The event system, like everything else, is really just a thin layer over SDL's—so for platform-specific issues in PyGame it's commonly the underlying SDL that's at fault. Notwithstanding that SDL is fairly well-tested, but Mac tends to be a lower priority for testing, on account of Apple making game development on OS X difficult in a variety of ways. My guess is that it's a minor hiccup in older SDL on MacOS that no one ever noticed before, additionally because MOUSEBUTTONUP is often ignored—in which case you're most likely to get closure from the SDL community, unless someone here feels like C spelunking.
Maybe `pygame.event.clear()` would trick the event system into behaving better on startup?
Ian