Trapped on top of the world!

[nfitch]

Repro:
Keep going down/right until you reach this room:

Swim down. Fall through many blank screens. End up trapped on top of the world!

[mxr]

Hit something similar as well - by swimming down far enough in a room, eventually I fell through the map, and ended up in a room which seemed normal, but left me stuck in walls if I tried to move left or right.