Its been rare, but I've seen the Pope do such things. Once, as Hungary, the holy father attacked venice. Not expecting the manuever, I withdrew to the keep. The next turn, I took Venice back, only to be warned by the pope to stop agression against him. Two turns later, he invades Venice again. This time, he's repulsed. What do I get for defending myself? A big, juicy, double order of excommunication! Now that's really frustrating.