Debug mode has bugs! If you place a minecart on a floating debug rail, it starts barfing rails until your fps goes to 0 and stays there.
How to do:
1. Create a debug world.
2. Get in Creative Mode.
3. Get a Minecart. (JUST a plain ol' Minecart!)
4. Place it on ANY rail.
5. MInecart pukes rails.
6. Your framerate goes to 0 and stays on 0.