For PC users, it's the Blue Screen of Death. For Mac users, it's the Spinning Beachball of Death. Here's one common problem for iMac users, and the solution.

I have an iMac running Snow Leopard (Mac OS X). It's been running well, when I'm not using it that is. Of course I have managed to cause individual apps to crash and the sound card to stop working, and there was that one unfortunate incident when it couldn't find the operating system when it rebooted, but when left to its own devices, sitting there doing nothing but showing my pictures screensaver, the iMac has been fine. That is, until I started using Time Machine, the built-in backup application. As soon as I configured that to start backing up my computer, whenever it goes into the screen saver and stays there for awhile (an hour or more), I can't bring it back out of the screen saver. I get the spinning beachball of death, and it stays there until I run out of patience. The handy CMD-OPT-ESC trio doesn't "force quit" whatever is crashing, I have to hold down the power button to shut it down.

So for all you out there who are in the same situation as me, here's the fix: disable the Energy Saver. Found under System Preferences >> Energy Saver, set both the computer and display to never sleep, and disable "Put the hard disk(s) to sleep when possible." That's all it took for me, I haven't encountered the spinning beachball of death since.