A thing that makes my brain hurt (and will do the same for all of my fellow sock-mating fans).
I bought a set of 4 pairs of black trouser socks recently. I didn't realize at the time, but they all have *slightly* different patterns, that are only obvious when stretched under bright light. They have been driving me INSANE trying to keep pairs together. The probability of anyone besides me knowing that I would be wearing unmatched socks is minimal, but that knowledge causes me intense mental anguish. I have taken to safety-pinning pairs together after taking them off, so that I can be sure to keep them together.