1) After a week long vacation I stayed at my best friend's house for a few days because I was sick of my family. In the week I was gone, he had just started to go out with a new girl and we all went to a movie, Sin City, so I could meet her. It didn't take long in the theater before they started sucking face that I just got disgusted and tried to sleep and ignore it. For the record, this was the case for most of their relationship - them making out nonstop and me and his other friends trying to ignore it.
2) Same friend, different girlfriend. After we became roommates, his girlfriend flew in from out of state to visit. After a little while of trying to sleep, I could hear 'em - actually just her. Most unattractive noise ever.
3) Made a dumb blonde joke in front of my blonde aunt unintentionally. Then out of nervousness, blurted "Oh... no offense!"
4) Called a kid a "faking cripple" in middle school because he used a motor chair but walked fine. Didn't realize he had a disease that prohibited him from walking long distances.
5) Also in middle school, teacher said something I disagreed with so I went to say "But doesn't the book say..." but she kept talking and all I got out was ""Ba...!" as in the sound a sheep makes (Baaah!). So everyone made sheep noises at me for the rest of the year.