Crashed the game with the Green Gun [CAUTION: SPOILERS]
So. I feel really special now.
After noticing that you can "farm" blocks with the Green Gun by making an enclosed shape, I thought the only logical thing to do was to try to perform the spoiler'd task... in 3D. After all, the Green Gun still seems to have the properties of the Blue Gun and once there's a block at a certain point, you don't need a back wall...
Well, apparently the game didn't like that. Trying to do a roughly 10 wide x 10 deep x 20 tall (didn't really count it out) 3D "farm" froze and shortly thereafter crashed the Unreal Engine.
Is this an unofficial achievement? A bug? Something the Test Team never thought of?