Grouping low-level instances

I made an attempt to get a group together for the sunken temple. After a minimal effort I founded a group and went out to the Swamp of Sorrows to summon everyone. Spot-on, everyone shows up and we head into the raid. Getting in is easy, no problem, but when we start getting down to it, our feral druid tank can't hold aggro from the healer. The healer (a paladin) can't hold aggro from me when I heal and the mobs begin bouncing back and forth. No problem, one guy drops due to an emergency and we find another, higher level character to come in. Same thing, even with a level 60+ tank we can't keep agro off the healer. So the instance group flops. Quite an awful night for doing an instance apparently. What's the worst grouping experience you've had?