Excellent list, David, but the Death Lends a Hand one could also have gone like this:

Brimmer: "I lost my own contact lens and was looking for it."
Columbo: "OK, show us the prescription." (And not one for Murder :) )
Brimmer: "Hommina hommina hommina"

David, that was a masterful summary of "Columbo loopholes". I just want to comment on one of them:

Dagger of the Mind "Columbo put the pearl there" (Actually attempted).

It's always been my impression that Columbo had no intention of letting the pearl be used in evidence -- he promptly and candidly 'fesses-up that it was a plant. The final "clue" in this case is the confession.

There is ample precedent for cops deceiving a suspect to get a confession, and it is considered fair and legal. Probably the "oldest trick in the book" is the cop telling a guy "Your buddy already told us all about it, and he says you were the trigger-man, so you might as well talk to us."

I think that "Any Old Port" presents a similar situation, where Columbo's "clue" is hardly conclusive, but serves well enough (together with Adrian's fear of marriage!) to get a confession.

"I only confessed because I mistakenly thought there was good proof of what I did," is not much of a defense.