I was on version 14w05a trying to make a machine that used smoke particles but when I put in the command and flipped the lever, my game froze completely, and nothing was possible in the game. I had to force quit the game to get out and when I reopened the game, I tried to log into the world but when I tried to log in (After it said "Building Terrain") it kicked me out and put me back on the title screen.