It has happened to me many times, where I will happily go up a few levels in a game, and then for some reason check a rule and realise something I have went up in power as I levelled up... which I totally forgot about.
most recent example being certain skills at 10 ranks increase their bonus (with a feat). Just totally glossed over that..
So what weird rules/abilities have you found out about, that should have changed with level and you didnt??