As i posted on a similar thread... we were about to go to wave 20 with 1 zombie left to kill when we noticed that the body fell down. We started to mess around with it then i went back to my glitching position and as we were about to kill the crawler the screen turned white and we restarted to wave 1..
Or you can just make it to rouns 25 plus and right when dogs start it will fall cause of the thunde ri have figured this out and makes the dog glitch a lil harder to do