MacOS X Spinning Ball of Doom in the notifier area
While I was setting up my new Mac and futzing about with it in general, I stuck in a USB flash drive that had been formatted with ext3. Needless to say it didn't mount it or anything, and I removed it shortly after. I then noticed that if I moused over the notifier area in the upper right corner of the screen (where the date, volume, Time Machine, etc. controls are), the Spinning Rainbow Ball of Doom showed up.