According to Steve Beverly's site, Johnny Carson was supposed to be on the panel of the notorious one trick pony called You're in the Picture. Apparently he showed up to do the runthrough, but once he saw the concept, he immediately bailed out.
I wonder what might've been if Johnny had gone though with it! Just as Jack Benny joked for years about his movie flop "The Horn Blows at Midnight", Johnny could've milked a lot out of his association with You're in the Picture on The Tonight Show.
I think Johnny could've made You're in the Picture a more receptive show, but then everyone would remember Johnny, and how he stole the show, and not Jackie Gleason, who would be trudging through the show trying to salvage it. Johnny Carson had an art of recovering from a bad joke or a bad sketch on The Tonight Show, and surely he would've been using his time on Picture trying to save the show from its inevitable doom. Just as Johnny Carson would immediately bring any failing sketch to a halt on The Tonight Show, Jackie should've stopped the game at some point... but then we'd be deprived of one of the longest apologies ever.