When I was sixteen, one of my dad's best friends came for a visit with his Filipino wife, whom I had never met. Towards the end of the day, my dad called me into the living room and told me I had some explaining to do; everyone was staring at me and I had no idea what I'd done. Apparently, the wife thought I was a racist because:
1) I barely spoke to her the whole time she was there, which she interpreted as me being cold and standoffish, when in reality I was just a really shy teenager.
2) At one point she asked me where the bathroom was, and I told her she could use the downstairs one; I thought I was being considerate, because this way she wouldn't have to go all the way upstairs, but she interpreted it as me saying she wasn't good enough to use our upstairs bathroom.
I was completely distraught and shaken up over the accusation of being a racist, and I apologized and explained these things, but no one seemed to accept my reasonings. I still get a little upset when I think about it to this day.