In high school, I used to think that the lead singer of Yaz (aka Yazoo) was a dude and the lead singer of Bronski Beat was a lady. You'd think that MTV would have cured me of that particular malady or that I would at least come across their names somewhere and reconsider my delusion, but no. And as teenage assumptions are sometimes damn hard to shake, even in the light of evidence to the contrary, it took me many many years to realize I'd had them reversed. Alison Moyet and Jimmy Sommerville, I formally apologize.
Also, even today, this still happens from time to time.