This is my fucking thread.
My brother got a haircut when we we kids. Mom asked me in front of him if I liked his new look. I said, "Yes. It looks like a girl hair cut." He cried. I got yelled at. I actually did like the haircut.
Again as a kid, I said to a friend of my mom's, "WOW, you look just like this girl I know. She's really ugly, though." She did not take it well.
In middle school, people always said that my best friend and I looked like siblings. We had this mutual friend who was sitting with us in class one day. We were joking around as usual, and best friend suggests that mutual friend could be our brother, except for the fact that he looked absolutely nothing like us. So, I said, "Nah, he's adopted. He doesn't count." Mutual friend went wide-eyed and pretended to slam his head down into his arms on his desk. I laughed pretty hard at his commitment to the act, that is until my best friend screamed my name and told me that he actually was adopted. I was in disbelief until I realized our mutual friend wasn't pretending, he was actually crying and pretty fucking upset. I apologized profusely. Everyone in the class heard this happen.
This polite, but super elderly gentleman started chatting me up outside a shop. He complimented my sweater. I said, "Thanks... it's... really old though" as I looked him directly in the eyes. He gave me this disgusted look and walked away. I was confused for a few minutes until I realized... He probably thought that me stating my sweater was old was a subtle way of hinting at his old age. Oops.
In my apartment building, on my floor. This guy popped out of the elevator right in front of me. He stopped me and started speaking to me in French. All I could catch was that he was asking if I spoke French. I lost my nerves and said, "Sorry, I took Spanish!" Then I turned and hurried away while French guy confusedly watched on for the entirety of the shameful walk to my door.
Classic "Your Mom" comeback to someone whose mother was recently deceased.
I'm positive there are more that I'm not remembering right now. I might be retarded.