The Fourth Crusade. You don't have to be a Byzantinophile to know it was one of the biggest disasters in Western history. Not only did it horribly backfire later on with Turkish conquest but the fact that it was supposed to be a Crusade really makes it a huge pile of dirt in history books. Huge part of the blame goes to Byzantines themselves of course but the bloody Crusaders were the last people I'd expect to do something like that.
Also, Valentinian III murdering Flavius Aetius, and the shit-for-brains who thought Ricimer would make a great Magister Militum.
but but renaissance!!