The swapping of positions of "Open Link in New Tab" and "Open Link in New Window" in the right-click context menu in Firefox 4.0.
Recently created documents in the PDF "Open" standard that can only be opened in Adobe's reader, and only in the latest version of that, at that.
The fact that MS Office's installer in OS X is too stupid to kill OS X's software updater despite the updater being smart enough to start the installer in the first place. (It's not the fact that the installer can't handle it per se -- it's the "Sheesh, would you shut down the updater already!" way it sounds like it's blaming you for the problem.)