Timrod recently posted about the arbitrary way invisibility disappears when you attack something. I'd forgotten about that one. I think it would be interesting to list other wuh? rules from older editions of D&D.
Don't worry about association or whether it's possible to come up with some explanation, just think back to when you were a fresh-faced youngster coming into the game and you read a rule and thought, "that's weird."
And nothing newer than 2e, we beat on the newer editions enough already.
I'll start off with the way psionic powers would often have class restrictions-- like cell adjustment. Wait, I have this awesome mental ability to heal myself but it's more powerful if I'm a mage than a thief?!