Name something you do that unintentionally annoys people

I am very bad with faces--if I see an acquaintance out of context (for instance, someone from work in the grocery store), sometimes I don't recognize him or her. I've even done this with family members when I've run into them unexpectedly. People think I'm being aloof and snobbish when I don't see them and greet them appropriately, and refuse to believe me when I claim that I
didn't recognize them right away.