In the secret room above the very end of the waterfall cave (past the masks on the ceiling), there's a mask hovering at the top of a ladder through the floor, and you have to position a butterfly to go down it. If at any point the butterfly drifts out of range of the mask while you're down there, it's impossible to call or push it back to leave. Rewind song does not let you escape because there's a checkpoint directly at the bottom of the ladder. Warp song does not work because...I'm not exactly sure why. The gold light hits the checkpoint, but Toki Tori just sits there and shivers instead of getting picked up.