I hate it when I'm playing a game and Sticky Keys decides to popup when you least expect it. No I'm not disabled FFS, Go Away- Only to have it crash when you try to Alt-Tab back to your game.
Or related to this, when a game uses 'CTRL' and 'ALT' as the primary and secondary fire keys, but switches back to the windows desktop to open the start menu if you accidentally hit the windows-key in between them.
Back in the DOS days, I remember mortal kombat on the PC. You could do a two-player version on a single computer/keyboard (poor keyboard!)
You could define your own key mappings -- but it didn't complain when one player had the 'CTRL' and 'ALT' among their keys, and the other had 'insert' and 'DELETE' among theirs. Now imagine 'ctrl', 'alt' and 'delete' being repeatedly hammered on during a DOS game. Oops. :-?