I found a glitch on the DMU servers: People can go up to a crowd of mobs and do /rtp then /back  ; When they are back all of the mobs have vanished. This can be abused to clear underground cities, Dalek asylums and a lot of things in general.  Here's a video for an explanation: https://www.youtube.com/watch?v=YRANisr-wPo&feature=youtu.be