The "famous" Banana Sketch, written by Gags Beasley, was a controversial topic in episode 114 of The Muppet Show. Kermit the Frog knew nothing about the sketch, but everyone else, including guest Sandy Duncan, considered it absolutely hilarious.
Nobody could believe that a showbiz professional like Kermit had never heard of the Banana sketch. But every time Kermit tried to find out what it was, everyone would crack up before they could finish it. The furthest anyone got was "These two bananas are walking down the street, and one banana says..."
When The Muppet Show writer Jack Burns hosted Saturday Night Live in 1977, he played Gags Beasley in a sketch in which he himself claims to have written the Banana sketch. "I got three thousand bananas -- three thousand! -- and we hung 'em all over the studio," he says. "It woulda worked, too, but it was radio."