Quite literally! I once used a multiple followers mod and invited Vilkas and Marcucio (you know, destruction mage, lives in the Bee and The Barb in Riften) to get through a really annoying dwemer ruin, and the moment I turned my back on them, they started fighting! No matter what, Marcucio always killed him with ease, and then I kind of got mad and killed him... it's a long story....