Your quirk (insert quirk) can go out of control, and one day it put you near-death. It changed your energy flow in your body, causing damage, but without that change you'd be quirkless! Unfortunately, the doctor treating you didn't know this, and as a part of your surgery, changed your energy flow.

Doctor: Your energy flow was a little wrong, so I made it normal! It might feel a little weird, but no more internal damage! Recovery girl: Horrified Bakugo, Todoroki, Izuku, Uraraka & Kirishima: Angry, shocked, worried, worried, shocked