My obsessions...
Memorizing pi - I got bored in math class in high school. Looked over at the wall, saw 100 digits of pi written on it. Also looked in the math text book, and noticed there were a looooot of digits of pi. Then on my calculator, it only had about 8? I memorized those, then noticed that it rounded up the last number. Then I decided to look into it more, and then sooner or later, i had ~82 digits of pi memorized. I looked back at the wall that had 100 digits written on it, and that's when I noticed A MISTAKE. Yeah, nothing special. I kinda recite it if I'm waiting forever for something, then start singing randomly.
Cows - moo.
Cheese - made from milk, which comes from a cow.
Ponies - Didn't see that one coming.
Video Games - Mainly JRPGs, Nintendo games, 'Tales of' series, some Sonic games...