Using /give to give a player a nonexistent item causes a crash. I was trying to give myself a Minecart with Command Block, and I forgot the item ID, and typed 488 instead of 422, and the game crashed. I remembered that, when I was trying to find the ID for the aforementioned Minecart variant, I tried 409 and it said no item with that ID existed. I tried it again in 13w39b, and I got a crash.