Each time one of the electric mixers are dropped into a vat of water, the unit explodes *before* it touches the water. This is most visible in the sequence when it is dropped into the bath in the electrocution scene.
When Columbo helps Dexter in the cooking show, watch his shirt sleeves: Columbo rolls up his left sleeve, then his right. Then, half a minute later, the left sleeve has neatly rolled itself down, and even re-buttoned itself at the cuff, forcing Columbo to unbutton and roll it all over again.
When Columbo is on Dexter's cooking show, the amount of lemon juice in the measuring cup varies inconsistently between shots. (And as an aside, one viewer who tried it out says that the recipe they use for the sauce also contains far too much lemon juice.)
When Columbo is interviewing Lisa at her apartment (who was engaged to the murder victim), in one shot she is holding a cover-up sweater. The camera then changes to a close up of her, and she is wearing the sweater.
While Mrs Peck is reprimanding Columbo for dropping cigar ashes on the floor, Columbo grabs a tray and knocks over a pitcher. As he grabs the pitcher with his left hand, he is also holding the cigar with the same hand. In the next shot he is holding the cigar with his right hand.
When Columbo is on Dexter's cooking show Dexter helps Columbo to put on an apron. The strings are too long so he leads them from the back to the front and ties them at the front. After the show Dexter helps Columbo to take off the apron and unties the knot which is at the back now.
When Columbo is talking to Dexter backstage after the cooking show, Dexter hands Columbo is suit jacket and coat. During the head-on shots of Columbo as he talks to Dexter, he his holding his arms and jackets low, just above his waist. During two scene cuts to a side-shot of Columbo, he is holding his jackets up against his chest.
(at around 1h 7 mins) When Martin Landau is playing both twins on the same screen - one in a blue suit and one in a brown suit - they both follow Columbo out of the room. Martin Landau is in the blue suit, and seconds later followed by a double in a brown suit, who looks like he's squeezed into the suit two sizes too small and wearing an obvious wig.