If you enter from Mossystep and walk to the far left side of the screen, you can see the top of a rock ladder, but it isn't reachable from the ground. It's behind a rock, and you can't walk up to it. If you ascend to the ledge above (with the mailbox on it) and position yourself above the ladder, you can climb down it, but you can't leave the ladder. The only way to go is back up.
Maybe it's meant to entertain us more simple-minded glitches. :)