I participated in a concert of the Christmas portion of Handel's Messiah. It was well - sung, well - played, and a wonderful time was had by all.
We had two performances, and the host church fed us in between the concerts. When we were in line for the food, I heard a voice I'd hoped never to hear again: "Hey! Didn't I sub for one of your classes?"
The voice was the voice of a man who was, at best incompetent at teaching, and in my honest opinion, incompetent at managing basic decency to his fellow human beings - when a national tragedy occurred, he made an awful joke about it, and was surprised when no one laughed. He was finally fired after he called the kids he was teaching "absolute retards," (I apologize, but that's what he actually said) and that only because a kid got it on tape.

My first instinct was to bring up all of the things he did, but I managed to shove Evil!Violin back in her cage, and I said, "I'm sorry; my memory's foggy. I really don't remember you."
It was the only thing I could think of on the fly that wasn't completely snarky and rude. I couldn't refuse to answer him, as that would have looked stranger than my responding, but obviously, "Oh, yes! You're the teacher who was fired for calling kids intellectually disabled!" would have gone over like a lead balloon and sent me straight to Ehell for quite some time. And, really, what would I have said politely after that if I had affirmed that he had been my substitute teacher? I can't think of anything that would be, knowing my personal history with the man.
My question is: Is it rude to pretend not to know someone when you really do to avoid a very awkward situation?