One of the Warp Whistles in Super Mario Brothers 3 is hidden in an unusual way: to find it, the player has to hold down the down button on a specific platform until Mario falls into the background of the world, at which point the player needs to run behind the normal exit into a secret room.
This is a pretty nifty secret, but I have absolutely no idea how anyone ever managed to figure it out. To the best of my knowledge nothing in the game ever indicates that this particular platform is special, that holding down the down button would ever have this sort of effect, or that it's even possible to get into the background.
How did the game designers ever expect anyone to figure this out? Alternatively, how did players first discover this?