I have mentioned this elsewhere online but I was once attending a talk in the computer history museum for which I had turned up early. An elderly gentleman took his seat right before me - it took me a while to process that it was Knuth (in reality, it was less to do with processing and more with accepting!).

Somewhat recently we spotted him at a Hitchcock movie festival at Palo Alto, which my wife and I were attending.

Random run-ins are surreal :-)