Ack, the secrets in Painkiller on some levels were pure evil, actually almost offensive platformy jumping puzzles.
There's one level where you have to find 7/7 secrets to unlock one of the bonus cards, and there's just no way you'd find them without a FAQ. Most of them being 'jump into the fatal water, but land on an invisible ledge' kind of thing. One involved actually glitch jumping up the side of a wall, jut for starters, then strafe jumping to gain stupid speed, then jumping around an impossible corner. I had to abuse the quicksave so hard to finally get that one!