Charles Hoskinson explains the "poisoned transaction" Cardano hack that took place back in November, and how it split the chain in two: “I don’t wanna have figure out, like, ‘How do we reimburse all these guys?’ They were honestly following the protocol, their nodes didn’t understand that there was a split.”

Source: https://decrypt.co/videos/interviews/sewi5umu/cardanos-november-hack-explained-by-co-founder-charles-hoskinson