I don't know if I'm missing something big, but getting Perl, Apache, PHP, PDO and MySQL to play nice on my Mac OS X 10.5 install on my Intel Core 2 Duo (Penryn) MacBook Pro hasn't been easy. This is partly thanks to Apache being compiled with x86_64 support, and Perl with i386 only.
The Flight Tracker dashboard widget that comes with Mac OS X 10.4 (Tiger) seems to have a fairly major bug. However, this bug seems to be the result of something quite obscure to do with timezones, and it only seems to manifest itself in the UK during the summer.
A friend of mine is a recent newbie Mac user, after finally succumbing to the shiny white lure of the MacBook. As an expert Unix user, he was missing the old X11 colours list, because he can't have his terminal set up with a horrible shade of puce without having to work out the RGB values for it.