I'm not talking about like 10.4 to 10.5, I'm talking about the 10.x.y releases. My vote goes for 10.5.6. 10.5.5 was pretty much working fine, but 10.5.6 seemed to break tons of stuff. Phantom unread messages in Mail, not being able to quit Mail, iTunes not even being remotely stable, Safari randomly ignoring cookies, the list goes on. Its especially bad after Apple criticized Microsoft for not fixing Vista..... However, I've only been in it since the late Panther days and have only really paid attention to the updates since the late Tiger days, so I am curious as to what software updates have caused the most headaches for you?