Years ago there were a couple of (female) Head of Departments that were frenemies. They actually worked really well together but there was an undercurrent of rivalry there. That year at the Christmas do they were friendly enough to start off with but a couple of bottles of vino later the gloves came off - one of them accused the other of giving a member of her team a BJ in her office, she totally kicked off and the pair of them ended up on the dance floor rolling around trying to pull each other's hair out. It was completely nuts. One of them later resigned and HR released a policy of how to behave when on work socials. That was the last year the company held a Christmas party.