Whenever I double-tap the Control (Ctrl ) or Command ( ⌘) keys, the current window is minimised into the dock. Annoying as it's pretty easy to trigger this completely undesired behaviour.
I'm fairly sure this only started happening within the last few minor OS updates; I don't believe I've installed any sort of new keyboard-shortcut-enabling software (I installed Quicksilver about two years ago, but that's all).
Anyway, I can't find anything in the Keyboard shortcuts system settings, nor could I find any good search results about this particular problem.
This is with Mac OS X 10.5.8 on an early 2007 MacBook.