Generally if you are inside of something you shouldn’t be, the game will pop you out on top of it (“upwarp” you). This can be abused in several ways.
The most well known / prevalent upwarp is this one. It was discovered by Cheftoad on 2018-5-18 in Waterfall (original video & Discord saga). Thermospore made a tutorial for it. It can be done anywhere that same geometry shows up, for example in: