At the end of every episode of the old game show, What's My Line?, there was the famous Mystery Guest segment for which the panel would don blindfolds and attempt to guess the identity of some celebrity. Over the years, hundreds of stars from the world of show business and sports signed in, then attempted to disguise their famous voices while answering the panelists' questions.
The big achievement, of course, was that the show managed to get someone every week. It was done live for most of its existence so it was not possible to delay if someone didn't show or showed up late or even showed up drunk. The producers were quite prepared for this to happen and they had two emergency plans if they were suddenly caught without a Mystery Guest at the crucial moment. One was for the show's Executive Producers, Mark Goodson and Bill Todman, to go on as Mystery Guests. The other was for the show's host, John Daly, to pretend to be a Mystery Guest.
In the show's eighteen year history, they came close a number of times to using the first of these fallback plans. At least twice, Goodson and Todman were backstage, waiting to "enter and sign in, please" when the actual Mystery Guest made a last second appearance. (Goodson and Todman did go on as Mystery Guests a couple of times for anniversary episodes but these were planned, not because someone hadn't shown up.) The idea of Daly as Mystery Guest occurred to a lot of people over the years and it was constantly suggested to the program staff. They agreed it would be funny but wanted to save it for the night when it might be needed.
When it came time to select a Mystery Guest for the final broadcast on September 3, 1967, someone realized there was no point in saving the idea for the future. The show had no future. It was also, obviously, appropriate for the last show. So Daly pretended to be a Mystery Guest, attempting to stump a panel that consisted of Martin Gabel, Arlene Francis, Steve Allen and Bennett Cerf. Let's see how he did…