Posted by: Lava1964 on 2009-03-19 
Let's see now...

Bennett Cerf was a mystery challenger on the syndicated version of WML in 1968.

John Daly was famously the final mystery challenger on the CBS version in 1967.

Steve Allen was a mystery challenger on several occasions.

Fred Allen was a surprise mystery challenger when he was supposed to be home recuperating from an appendectomy.

Ernie Kovacs was a mystery challenger during his short stint as a semi-regular panelist.

Dorothy Kilgallen was the mystery challenger here.

Only Arlene Francis seemed to miss out on the fun of sitting opposite the WML panel!
Posted by: Lava1964 on 2009-03-19 
Too bad Bennett Cerf was on vacation that week and missed out on the surprise!