What Lies in the Multiverse

This can be done fairly early in the game, it was one of the first couple achievements I unlocked. Look for a spot where there's a difference in wall/ground layout between the two different worlds you can switch between (like a wall or crate in one world that isn't there in the other). Position yourself in that exact spot and switch worlds with ; you'll flip to the other world and be stuck inside the wall and flashing. You'll likely do this a couple times throughout the game on accident, but either way the achievement will unlock the first time it happens.