Yesterday I noticed that my music was just cutting out every few minutes (I run a USB cable to my stereo, which lets me use iTunes and a decent sound system). I could "fix" it my unplugging and replugging the cable. So I did the obvious thing - ordered a new cable.
Then this morning the Mac wanted to reboot, so I actually looked at the back. Seems the USB cable to the stereo was hanging a bit out of the socket (meaning, whenever there was a bit of bass, it went loose. Doh. Switched the cables around, and it all works fine - the one coming from my time machine drive into the same socket doesn't look loose.
So I have another USB cable lying around now :)