While pair programming the other day, I noticed that I was getting distracted and jumpy. It took a few minutes to notice why.
My teamate's PC has an old disk drive in it. When the drive seeks, which it did only periodically since we were doing most of our work ssh'd into another box, the drive makes a particular "chu-chunk" sound that is nearly identical to the "someone within earshot has just acquired a big-assed weapon and it's time to run like hell unless you want to die" sound from Quake II. I haven't played Quake in several years, but apparently some of the game's sounds still trigger a twitch reaction.
It's hard to concentrate on code when you're about to get fragged.