Play through Super Mario RPG and you'll eventually discover that there are hidden treasure chests all over the place that contain some goodies. You even get an item that 'dings' when you enter an area that has one, making it easier to find them. There's one room, though, that gave me fits for a while: the hallway to the Mushroom Castle.
It turns out that you only have one shot per game to get this box. When you talk to the Toad and he starts walking down the hall, you have to jump on his head
And then jump on top of the door
Where you'll find the hidden chest
This is the only chance you'll have to grab it, and if this is the first time you're playing this game, you probably won't even know that you're supposed to be looking for hidden boxes. It's pretty devious, really.