Almost got into trouble
At school, we used to have a teacher who was the spitting image of one of my mates. People who didn't know better would swear they were related, or even get one confused for the other.
One day I walked into a room and, seeing my mate sitting on a table, decided to greet him with a friendly backhander to the head. Except it wasn't my friend at all, it was our teacher. Luckily I managed to stop myself just in time, but a few nearby people were probably quite confused as to why I made such an elaborate hand motion just to scratch my head.

Also, in an anecdote only vaguely relevant to the question, I was once introducing a couple of new uni friends to one I'd known for a while. As the two of us walked over to the bar he turned to me and asked "Are those two sisters?"
"Erm, no." I replied.
"Oh. They look very alike." he said in a surprised tone.
"How exactly?" I asked, not seeing any resemblance myself.
"Well," he said, "they've both got glasses."
(Uncle Comrade, Sat 2 Jun 2007, 18:35, Reply)