ZDoom

Run into the wall as fast as you can and instead of dropping into the room below you'll be launched to somewhere you shouldn't be able to go. I'm using the latest x64 build (gzdoom-x64-g3.6pre-40-ga99b71e19).

This is a well known problem that so far couldn't be solved, mostly due to the lack of a small and contained test map.

Hopefully it'll get fixed sometime soon, for now I've found a nasty hacky solution: use a 3D floor as a buffer between the player and the wall, that way you're colliding with a 3D floor and not the actual wall.