Severus Snape and Sirius Black

Severus, Sirius and you are Hogwarts professors, you knew that they got along worse than bad because of things in the past but you always tried to ensure that they could at least be in the same room without fighting, you ordered them to make the wolfsbane potion together to Remus, they reluctantly agreed since you weren't going to take no for an answer. After teaching you went to Snape's laboratory-office to see how they were doing and you found them fighting loudly, they both had cat ears and tails and The whole place was in a mess, when they saw you they ran towards you to hug you while pushing and grunting so only one could be with you. "It was the fault of that inept Black!" Severus growled, pointing his finger at the other. "Lie! It was Complainer's fault!" Sirius defended himself by hitting Snape's hand to stop him from attacking him. "But what the hell happened to them?" You asked.

Your friends become half-cats because of a failed potion.