Subtle things like:

Blueprint for Murder: Elliott Markham leaving the radio tuned to a classical music station.
Agenda For Murder: Oscar Finch taking a harmless bite of cheese
Columbo Likes the Nightlife: Justin Price and Vanessa Farrow's phone calls to each other
Make Me a Perfect Murder: Kay getting dry cleaning dropped off at Mark's house.

I'm sure there are a ton of others; take it away folks, let's see how many we can come up with.

Two that spring to mind involve cigarettes.

In 'Caution Murder Can be Hazardous to your health' the killer forgets that smoked cigarettes have a nicotine stained filter.

In Strange Bedfellows, the killer takes his cigarette butt out of the ashtray but forgets to empty the ashes. Obviously he wasn't aware that the car had been cleaned recently.

The Greenhouse Jungle: The final clue (the third bullet pulled from the dirt) is a clue the murderer never thought about.

Mind Over Mayhem: Dr. Cahill leaves a burnt match in an ashtray never thinking that it will be a clue. (Seriously, who stops in the middle of staging a crime scene to smoke a cigar?)

And why did Cahill even stage that scene in the house? He should have brought the body back into the lab where he had been working and where the heroin was which was supposed to be the motive. And why did he need to look on the index card in the file cabinet to see that Finch was filed under F... and then put the index card back into the file cabinet instead of dropping it into the acid along with the file itself? Cahill was running a think tank???

The parking garage ticket stub under the windshield wiper in Uneasy Lies the Crown

nice topic.

paul galeskos newspaper idea backfired on him for one.

The key that Greenleaf had made that he planted in Eddie Kane's apartment. He didn't know Columbo had the lock changed so Greenleaf's idea backfired and became a clue against himself.

Ken Franklin's open mail

And of course Ken depositing that blackmail money back in the bank right away, when he could've left it out for a while.
(From what you hear of his spending habits, there wasn't much of a point anyway!)

one of my favorite episodes is Columbo Goes to College. Justin Rowe has no idea that Professor Rusk called the restaurant to tell his father that he is going to be late for the meeting. Later on when Columbo finds out that there is a match in last name, he knows that he's on to something.

Franklin could have said that it was old mail that had been previously opened.

Galesko's failure to retrieve the discarded picture of his wife tied up followed by his idiotic declaration of how bad a picture it is to Columbo (Galesko is trying to set it up so that the police think a simple-minded individual committed the crime. It shows total stupidity on his part to first leave the discarded picture behind and then go into expert mode)

Even though he's sure by now that Paul is guilty, Columbo calling that ransom photo of Frances "a perfectly good picture" is a real faux pas! And Paul should have used that to feign some kind of anger at that remark, since he was supposed to be the bereft husband.

The feather from the down pillow on the cruise boat (ship) hehe

The typewriter ribbon with the letters from Jesse Jerome's note.

"Dust.. dust.. dust.. dust.. dust.. No dust."