What's the best comeback you've ever given?

Please describe the situation you were in, and how you dealt with it. 

I wish I had an epic story, but I think the one time it happened to me I was so shocked I didn't know what to say. Lame! I was inspired by an old GoodMythical  Morning (YouTube channel) though, where Link describes what he would want to say: "Sorry, could you repeat that? I want to see if you're stupid enough to say that stupid thing you said one more time."

It's not the smoothest line, but I think I'd feel better for saying it. :D