Janus' secretary gets a phone call from Mrs Stafford. She turns on the tape recorder and starts talking. She says "Mrs Stafford?", then a moment later we hear a dial tone, indicating that Mrs Stafford has hung up. She then plays the tape recording of the conversation. This time, we hear the dial tone begin before she says "Mrs Stafford?"
When Columbo finishes walking on the treadmill at the gym he enters the office to talk to Milo Janus. Just before he leaves the office he borrows a pencil from a black box on the desk. When the camera switches from Columbo to Janus the number of pencils change from three to one, and they are also pointing in a different direction. When Columbo puts the pencil back in the box there are two pencils, but the next angle shows three pencils as Columbo is heading toward the door.
After kissing his secretary, Milo's right hand moves from his hip to being interlocked with his left hand and back to his hip again. Also, his secretary's arms move between shots both before and after the kiss while she is talking to Milo.
(at about 18 minutes in) Just after Columbo arrives at the crime scene, on his way to to Stanford's office, he walks down a corridor. In the end room you see a chair is visible in the distance. Somebody is hiding behind it, and ducks down as Columbo approaches.
The bar used in staging the "accidental" bench-press is not a pressing bar, but a bent "curling" bar. That type of barbell is only used while standing and "curling" the bar to build biceps. It is too narrow for use in bench-pressing movements, which should have been an additional "flag" for Columbo. The wrong bar is never mentioned as a clue.
Milo places a barbell on Stafford's neck which is supposed to appear as the cause of death. Columbo says that the barbell weighs 180 pounds and Milo doesn't question him. However, that type of barbell weighs no more than 120 pounds.
After Columbo is Informed That Lewis Lacy Has Been Terminated by The Receptionist, he Calls The Contact Number & The Answering Machine States That he is Away From His Desk...Not Something You Would Have on a Home Answering Machine...
After Milo Janus has just killed Gene Stafford, he moves the body. As he moves the body of Gene Stafford, his feet are crossed, but when he drags him through the door to the underground garage, his feet are wide apart.
The goof item below may give away important plot points.
Columbo demonstrates that the victim's exercise shoes were not tied the way a right handed man would tie them, indicating that someone else tied the laces instead. However, in the second scene with the victim, he shows himself to be left-handed (holding a pencil).