In the warpgate area of Endless Gate, the player is able to venture outside of the intended playable area. It is possible to transverse the small platform behind it and see other areas of the map. Jumping off of the platform results in a death.
Apologies for weird crops, Steam didn't agree with me exporting screenshots, so I had to screenshot the previews and crop them myself.
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There are hundreds of these places in the game. Archers Line on the moon straight up has a hole in it you can fall through where a structure meets the ground.