What did you not realise until embarrassingly late?
I'm allergic to orange so I avoid anything coloured orange, just in case. I've only just learned that orange smarties are flavoured like orange - I assumed they tasted the same as the other ones!
My father didn't realise that Eeyore was called Eeyore because he was a donkey until he was well into his thirties.
What have you not realised until it's embarrassingly late?