when they hid a bonus stage in another bonus stage. Not only is it completely unmarked, but to get it, you have to "win" the worst possible price, a single banana, which then transforms into a barrel to bust open the wall. Even though the same bonus in a previous world did just give you said single banana, no barrel there. And if that wasn't enough, even if you know what to do, getting into the entrance is pretty finicky as well, if you're too slow the game will take control away from you and play the lose animation.
At least it's so dumb that once you learn about it, you'll probably never forget it in future playthroughs.