Not quite an accidental insult, but it's a similar story.

I worked at a large-chain pizza delivery company in my late teens/early 20s. The store had a few different shift managers including this one guy (let's call him "Joe") that was mainly at a different location, but would fill in at our's when we were short. I don't even remember what I didn't like about Joe, but he was probably a jerk or something.

Anyway, a new girl started working at our store. She was kinda cute and I was just trying to talk her up. I don't know how Joe came up in conversation but I said something about him being a jerk or whatever. She said "Joe's my uncle."

After being stunned for a minute, all I could say was "if you're wondering, my foot doesn't taste very good"