A long time ago, a woman at work sent me an email (using our work accounts) asking me if I knew a good place to buy a training bra.
I wrote her back asking, "Do you mean training as in, for young girls? Or do you mean sports?"
Except that I accidentally sent my reply to the person who wrote the email above hers! Which was actually a business email of some sort, that had nothing to do with training or bras. You can imagine the confused response....
Fortunately I didn't actually mention "bra" in my response, and it was a female I accidentally sent it to anyway. Still awkward and embarrassing though, that's for sure.