this particular piece of "trivia" is on the IMDB page for Beach Blanket Bingo.....how anyone who has the nerve to post something on that site proclaiming himself to be a movie "buff" can write something so stupid makes me hate the internet....
In the scene where Don Rickles is doing his comedy routine, everyone in the club is laughing, except Buster Keaton (who can be seen in the background) indicating that he didn't think Rickles was particularly funny (and didn't realize he was in view of the camera). May be a directorial choice but is interesting to observe due to Keaton being such a comedy icon.