My group and I started playing D&D when we were still in elementary school, we haven't grown up much since then. Over the years there has been alot of party infighting, in fact the only TPKs have resulted from player vrs player combats. Even when they aren't killing each other player pride and dickishness has shone through and made it difficult for the party to work together, (the party had a particular fondness for chaotic neutral..gah!)
Right at the end of high school the players decided they wanted to start a new game up, but with evil characters. There were a few evil wizards, a dark knight, and an evil ranger, one or two players were vampires. Funnily enough the party worked together during those sessions more than any other time we played. It was kind of creepy how well they all came together and razed a peasant village, they were genuinely going out of their way to be helpful to one another.
This summer when I head home the old group is going to get back together and start a party of lvl 10 evil heroes. They have some plans in the works for becoming liches, raising armies, and toppling the kingdom of Taldor from the Pathfinder setting.