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?
Best Answer
There were two hints that the warp whistle was there, and both of them were contained in letters from Princess Peach.
Between worlds 3 and 4 you get this message:
And between worlds 6 and 7 you get this one:
Although it's not a map and a sign, both hints together are enough to point the way to this particular whistle at 1-3. It looks like a basic way to reward a replay as you made it through the game. There are even less clues for the other whistles; one other letter makes reference to "east of the sand dunes" for the desert world whistle, and for the first fortress whistle, there's nothing to hint at it other than Super Mario Bros hid a warp zone in exactly the same way (plus as mentioned by Ryan, it was practically advertised in a movie).
Otherwise, intentional vagueness and cryptically hidden random-access secrets (like white mushroom house, and pirate ships full of money) was a good way to sell video game guides in the early Nintendo days.