The other day, I was replaying through the Dawnguard questline and I brought Serana to Castle Volkihar. She was walking down the stairs to Harkon, when she suddenly said "where'd you come from!?" and attacked her father. This started a long fight between the two while the other vampires watched yelling "A fight!"
I was laughing for a while because of how sudden and unexpected it was, until I realized that I was playing around with console commands earlier and must've made her aggressive towards Harkon.