Here's one that's always bothered me a little. Adrian Carsini buying a $5,000 bottle of wine, knowing that the $5,000 check he sent to Ric isn't going to be cashed by Ric. Maybe he wasn't thinking of that at the very moment the bidding reached that amount, and maybe he went around doing that kind of buying anyway (which was one of Ric's complaints), but it was still a little reckless.

Great examples already mentioned! Some of these others may be more about the killer’s character traits than “behavioral” clues, but are examples of Columbo watching the killer closely …

Dead Weight: Columbo’s insight about Hollister’s unbreakable bond with the murder weapon.

Now You See Him: Columbo perceives Santini’s gigantic ego, and knows he can exploit it, even to get Santini to incriminate himself by picking an “unpickable” lock rather than fail in front of an audience. (“I knew you could do it” - *wink*)

Mind Over Mayhem: Columbo sees Cahill’s love for his son, and (perhaps not Columbo’s finest moment) decides to use it against him.

The Conspirators: Devlin’s habit of scratching his bottles with his ring is, of course, a key clue.

Thanks for your replies. :)

In "A trace of murder", when the killer pushes the cream and sugar toward the woman. Columbo is so stunned he has to excuse himself and regain his composure in the men's room.

And Columbo is standing in the restroom and he thinks, in his best Yuban commercial voice, "Shera *never* takes cream and sugar in her coffee at home."

(Sorry, couldn't resist!)

I was visiting a relation some years back and they had that episode on the TV but the sound was turned down. I couldn't resist glancing at the TV from time to time and I saw that scene and I reacted to it bit like Columbo did! I hadn't seen that episode before.

I find that the thing which almost always intrigues Columbo initially is the murderer's apparently cast iron alibi which usually goes along with the idea that "it could have been anybody", except the actual murderer.