Three times now a portal guardian has fallen through the floor, once resulting in a wipe. Happens on both regular and heroic and things can get interesting when it does. There should be an achievement for dealing with it.
The first time it happened was on the portal to the far right in the front (when standing in the doorway). Had to kill a few extra adds and the guardian popped back up into place and was killed without further incident.
The second time it happened it was the far left portal on the floor. This time the guardian stayed below the floor. This was shortly after the first boss. We made it to the second boss, that etheral dude with the orbs of doom. Halfway through the boss fight things got hairy and we wiped. Didn't help that our healer wasn't on vent and had old mods preventing him from seeing party chat or whispers. Was pretty funny after we found out, he thought we were just being really quiet/ignoring him.
Third time it was the same far right portal again, this time the guardian stayed below the floor too. This time it was regular though and we had an 80 hunter just camp the portal while the rest of us continued. She continued to help whenever there were things within range to shoot.
Something to look out for when running a quick violet hold run that should take less than 20 minutes.
Still it doesn't top Azul'Nerub for the buggiest instance in the game.