What did you think you'd never have to learn again, and then had to?

I spent my whole life assuming that once I survived middle school myself, I'd never have to think about locker combinations, pythagorean theorem, or which table sits near the window that gets the good sun in September. Then I became a teacher and now I spend every day explaining how the Treaty of Versailles is actually still relevant. So what's the thing you thought you were done with that came back to haunt you?