A game that used to be played on the Tonight Show with Johnny Carson. Johnny would go into the audience and ask them for a title of a song and Doc Severinsen would say "Yes, yes, we know that one" and the play some Standard like "Misty" or "Baby Elephant Walk" and everyone would laugh and the audience member would get a coupon for a free steak dinner at Black Angus or something.
Okay, it doesn't sound particularly funny, and, by itself it wasn't. "Stump the Band" was essentially and exuse to let Johnny Carson interact with the audience (which was funny.)