One that comes to mind is in Death Lends A Hand where Culp pulls Columbo's tie out of the tomatoes, which was not scripted, and Falk ad libs about getting it on his tie.
I read that Falk had the "Where did you get those shoes?" idea for "The Most Crucial Game" and heard on an interview with Peter Falk that it was his idea for the final clue in "Agenda for Murder" years later.
I'm surprised there aren't more posts on this topic...ad libs.
I was going to ask the question...listing the tomato/tie scene as MY example...of an ad lib for sure
another I think is in Publish or Perish when Columbo has a hard time getting up after eating the chili in the restaurant ...seems like an ad lib.
in "Murder by the Book", the script has Falk saying the secret to a good omelet is "no milk, just eggs." In the actual show, he says "no eggs, just milk". It's either a flubbed line, or an ad-lib, I don't know which.
I heard Peter talk about this one......In "Any Old Port" Peter was on the phone waiting on hold and started whistling This Old Man, that one was an ad lib, it stuck and sort of became an unofficial theme.