My worst effort was putting a new spawner in our base for endermen. Already had a T5 skele, zombie & creeper spawner up there, added a new setup with pistons & wiring so I could drop endermen down the same chute into our base for leg-chopping. Unfortunately I didn't consider the whole picking up blocks part & some of our base still had dirt walls (temporary stuff) - as soon as I turned it on they started ripping apart the system on the way down and even worse they managed to grab a block that had the bundled cable for the off switch attached to it, turning on the zombie skele & creepers, and also open up an exit at the bottom so they could all walk out into the base.
Long story short our base got totally taken over by 4 tier 5 spawners worth of mobs churning into it. Managed to get out the door & fly up and get levers turning off all the spawners, but it took AGES to actually get our base back cause it was wall-to-wall mobs & terrifying amounts of creepers mixed in. My mate who shared the house wasn't impressed