Thancred <spoilers>

[Viridiana]

Changing classes didn't really seem that odd to me either. I mean, most people have more than one class leveled even if they main one. He was just screwing around on a secondary class while doing low level content until now. >_>

Except he seems to do it mid-fight, which players can't do. Unless you want to argue that he wasn't on anything's aggro table and was thus "between fights" when he changed, which I suppose you could argue.