Oh boy. I've got lots of these...
When my brother and I were little kids, my brother hit his head on something and got a huge goose-egg, and when my mom pointed it out to me I said, "That's gruesome." She got mad at me.
Another time, when my Nana first met my sister's husband, I pointed to him and said "There's the guy who knocked up my sister!"
One time, when I was in junior high, I started talking to a group of kids about a commercial for KY warming fluid because I thought it was a lotion that you put on when you were cold. I had no idea it was for sex.
Another time in junior high, I said "foshizzle mah nizzle" in front of this white kid, but he got really mad at me because he said he was adopted and all of his family members were black and that I was being really offensive. He could have been making it up though, because he was a major pothead and could have been high that day.
When my sister and I were the same size, she used to raid my dresser and take my clothes without asking. One day, I was wearing a new shirt and she said "I really like that new shirt". Thinking she was going to try to borrow it, I said "Yeah, it's too bad you'll never get to wear it." I really didn't think I was being all that mean, but my parents got very upset with me.
And the worst for last...
My sister had a miscarriage when she was with her now ex-husband. One day she said or did something that made me really mad (can't remember what it was) and I said "Y'know what? I bet you had a miscarriage because God knows you're too mean to be a mommy!" I felt extremely guilty about it afterwards and apologized a million times, and I still feel guilty about it to this day even though she forgave me.