The three battery problem
I've noticed that a lot of devices these days use three AA or AAA batteries. (The reason for this is most likely due to the increasing popularity of 3.3v electronics over traditional 5v systems, meaning that three batteries can be used in series instead of four in order to give the required voltage.)
However, AA and AAA batteries are only ever sold in packs of two or four, which means you always end up with one spare!
Also, if you use rechargeable batteries, a lot of chargers will only charge batteries in pairs!