The Men That Corrupted Hadleyburg - S2-E18
Audio problem: Final sound edits were incomplete on this episode when star Pete Duel committed suicide, so his dialogue is "looped" by another actor in several places. Though it's an excellent imitation, it is most noticeably not Duel's voice when Heyes and Curry are waiting for Harry in the opening scene, at the campfire with the Tapscotts, and when Harry tells them about the trial's outcome.
Any Old Port in a Storm - S3-E2
Audio problem: Pay phones always dropped your coin into an internal cash box when you finished your call. In the bar, Columbo makes two calls, and after the first, we hear the coin drop as it should. When he finishes the second call, we should hear that coin fall, too, but it doesn't. (00:34:05)
The Man They Called a Magician - S1-E2
Audio problem: Jigen shoots Pycal in the head with no results. He says that his gun is useless, but his lips don't move. (00:05:50)