I often use Cmd-Tab to switch apps. Occasionally (when the machine’s under high load?) it seems to lose the key release event for the Tab, so as far as OSX is concerned I’ve pressed Cmd-Tab and am still holding them down. The result is that the application switcher window appears, but rapidly cycles through all available app icons. I have to time my release of the Cmd key really carefully to try to catch the correct app.
If I continue to keep Cmd depressed but also hold down Shift, then the application switcher window toggles between whichever app icon it had last highlighted, and the one before, as if it were fighting itself processing both Cmd-Tab and Cmd-Shift-Tab simultaneously.
Another much less common bug is that occasionally some multi-touch features stop working. Specifically I often use the four-fingure swipe up to expose the desktop. This currently doesn’t do anything for me, even when I toggle that feature off and on in the Trackpad tab of System Preferences. Other multi-touch features such as two finger scrolling, secondary click and screen zoom continue to work, but pinch, rotate and the three and four finger swipes do not.
Finally, I often seem to have a swap file of over 2GB, which probably causes a lot of performance issues. Safari’s a common offender, but in fairness I do often have lots of windows with lots of tabs open. I could just upgrade my RAM, but I’m a stingy student. 4GB 1066MHz DDR3 (PC3-8500) – 2x2GB SO-DIMMs cost £142.60 from Apple, though £78.19 from Crucial is almost tempting.