The slide and doll glitch is a glitch which occurs in Pokémon Ruby, Sapphire and Pokémon Emerald.
Summary
When you walk on the same tile where a big doll appears on the screen, you're supposed to appear behind it, but if you ride on the slip of the slide and appear where the doll is, then you'll appear in front of it.
Steps:
1) Have a Secret Base with enough space (2x4 tiles and 3x2 tiles).
2) Place a slide, and then a big desk directly below it.
3) Place a doll on the desk, with the slip of the slide above it.
4) Walk on the slide, and when you go to the bottom you will be a layer above the doll.