A shapes patch which puts two different images onto a single texture. The picture you see depends on the angle at which you see it. Not perfect (by a long shot), but clever, and (to my knowledge) never done before.
A few wall textures have been replaced by lego-looking stuff, and the bobs have been replaced (mostly) by stiff-legged little lego guys (but don't kill 'em, they still use the original bob death sprites). Eh.