Donkey Kong Country Returns

One of the levels in DKC (I think it was Treetop Town) has a bonus barrel inside of a bonus room. So you enter one bonus barrel, start the room, and then there was another one.

Actually, it was a factory level. The bonus stage was match three, but if you matched three single bananas, you got a barrel instead. Pick up the barrel, jump into the right wall, and ... surprise, another bonus stage! If you messed up at any point (including JUMPING into the wall), you WON'T go into the bonus bonus stage, and you CANNOT get to it again, EVER!

Then there were the bonus levels that were just plain retardedly SNES-hard to find, such as trial-and-error jumping into holes, seeing only a few pixels of a bonus barrel while you're clinging to a moving rope, etc. But hey, I can accept those.