When I'm stumbling around trying to find the keystroke for lowering the screen brightness in quarter-segments (which happens to be Option ⌥+Shift ⇧+Brightness-down (F1)), I usually wind up hitting Command ⌘+Brightness-down, which puts my multi-monitor setup into mirrored mode, thus messing up my window arrangement and sizing.
I'd like to turn off this keystroke (which is, specifically, ⌘+Brightness-down and definitely not ⌘+F1. If someone can come up with a way to permanently disable display mirroring under all circumstances, I'll probably put together a bounty for you.
I'm running OS X Lion on a sufficiently recent MacBook Pro.
I'm not sure why they include mirrored mode in modern computers, anyway. It's never useful and it only confuses people. Don't tell me it's for presentations: any decent presentation software will have a presenter mode that everyone should learn to use and love.