Had a group on the 2nd half of MSV... hadn't even pulled Spirit Kings, one of the 4 mages in the party does the Ancient Dalaran Portal-on-the Banquet thing, and then starts calling out for the people that accidentally clicked the portal, and the other mages, to get kicked.
By the time all was said and done and we found the culprit mage, we'd lost 1/3 of the raid, including two healers, and we still hadn't even done a single boss pull. This took 15min.