When you wear your socks, they wind up stretched asymmetrically, e.g. your big toe sticks out farther than the rest of your toes. If you take a worn sock and put it on the other foot, it does not fit right.
Anyway, I nearly always manage to put the wrong sock on if I grab a pair I have already worn (no, not stinky old ones! ewww!). Why is this? It drives me nuts. What is the probability of this happening? I have two feet, and two socks, so it's a 50% chance that when I grab a sock it will wind up on the correct foot. But in my case, it's like 10% of time. Why why why?
Well, this morning I figured it out. I take my right sock off first and toss it on the floor. The left sock lands on top of that. Later on, I grab whatever sock is on top and put it on my right foot. So if the sock-pile is undisturbed, voila! Wrong sock on my right foot. Mystery solved.
Don't you feel better? I do.