- Germans Love David Hasselhoff: The German version, hosted by Robert Lembke, ran from 1955 to 1958. A second run was produced from 1961 to 1989, when Lembke unexpectedly died, meaning it ran longer in its second version than the CBS and Syndicated runs of the American original combined! It ran once more from 1999 to 2005.
- Harsher in Hindsight: On the April 4, 1965 episode, John Daly said to the mystery guest that night, Maurice Chevalier: "I just hope that when I am 77 (sir, if you will forgive me), that I have your ebullience of spirit, your perennial and wonderful youth, and that glorious confidence". Daly died four days after turning 77 in 1991.
- Hilarious in Hindsight:
- The interview with Sean Connery after he's identified on the 3 October 1965 episode. Now that we know he was tired of playing James Bond by that point, it's obvious that the whole time he's desperately trying to turn the conversation towards his other work, only to keep getting roped back into talking about Bond.
- Robert Morse puts on a fake deep voice to hide his identity when he was the Mystery Guest on the 12 March 1967 episode, which sounds quite similar to how deep his voice really did get in his later life.
- Informed Real Life Fame:
- Retroactive Recognition:
- As mentioned above, Colonel Sanders of Kentucky Fried Chicken fame appeared as a guest before he was a household name.
- The Sept. 16, 1956 episode opened with Jacques Cousteau as the first guest. Since Cousteau was not familiar to the panel (Cousteau had signed in as Mr. X), all of them were stumped.
- They Changed It, Now It Sucks!: Some fans of the classic series aren't too fond of the 1968-75 syndicated version due to the reduced emphasis on formality (panelists dressed more casually and were referred to on a first-name basis) and the demonstrations of new inventions (which was never done on the original series due to John Daly not being fond of the practice) making it too similar to I've Got a Secret, to the point where they nicknamed it What's My Secret Line?.
- WTH, Casting Agency?: The appearance of actress Pamela Tiffin as a panelist on the March 12, 1967 episode. Commenters on YouTube have called her one of the worst panelists in the history of the show, due to her poor game play and a number of on-air gaffes (including forgetting to introduce Bennett Cerf at the beginning of the show).
YMMV / What's My Line?