On multiple Macs, I have had the left/right balance of my headphones randomly drift. Apple says this can be caused by changing the volume while the CPU is under load, which doesn't seem to be what is happening to me. In any case, it's annoying.
Here's an AppleScript that will automatically reset the sound balance to the middle for the current output device:
There's also a $5 app called Balance Lock that in theory will solve this problem – I haven't tested it.