One of our warlocks stepped away on /afk and put himself on /follow to another player... who then jumped in the lava, with the warlock following right behind him. We summoned the other player out, and left the lock to flounder in the flow. And while the summon out of the lava was going on, one of our mages put a portal to Orgrimmar right over the summon portal, and three of our guildies ended up in Orgrimmar before they realized they were clicking on the wrong thing. Two fights later, our MT dragged Garr's last minion away from us back towards the lava (we had all crowded around him before he exploded just to send us all flying), and instead sent a few guildies flying into the drink. Basically, there was shenanigans aplenty in Molten Core tonight.
And this is definitely not the first time I've seen this kind of thing go down, either. Our mages love to give out level 1 conjured water, just to make us mana users angry. And way back when I was going through UBRS on my first time (on my gnome warrior), I got freaked out when Vael (who I later learned is a friendly NPC in UBRS) appeared and I was frantically told by my guildies to "tank the red dragon!" It took me a few seconds to figure out why my taunt button wasn't working.
Are you the same way-- does your guild prank each other mercilessly, or are we all just crazy?