When Columbo helps Dexter in the cooking show, Columbo rolls up his left sleeve, then his right. Then, half a minute later, Columbo again unbuttons and rolls up his left sleeve.
As he comes on stage at Dexter's cooking show, Columbo insists on tying his apron himself, at the front. When Dexter unties it at the end of the show, it's knotted in the back.
When Dexter puts the new mixers on the counter, Columbo is holding his jacket and raincoat on his arm with the raincoat on top. Dexter puts the mixers away, and the jacket is now on top.
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 the killer turns off the alarm, the switch is not in the same position it was when Mrs. Peck turned it on a few minutes previously.
When Columbo is interviewing Mrs. Peck about her TV going off the night of the murder, he accidentally pulls the tuning knob off the TV.
This would not "break" the tuning knob in any way. All one has to do is to simply line the cam on the knob with the cam on the tuner shaft and push the knob back on the TV.
When Dexter and Columbo are on the cooking show, the recipe calls for 4 eggs. Columbo breaks, separates, and adds 8 eggs to the blender along with lemon juice and pepper which are then blended in the blender. Once blended, Dexter pours the mix on a plate without baking it or cooking it in any way. Columbo then eats it. The recipe is for Hollendaise sauce, so no baking or cooking is needed.
When the killer goes to murder Clifford Paris in his bathtub, there is an electrical outlet right next to the tub, which would not pass building code for bathrooms in the U.S.A. as of 1975. But this episode was aired in 1972, so the outlet seen is not against regulations. Additionally, no previously-built home would have been required to retrofit existing outlets.
On the TV soap opera, the young doctor is first called Rocky, then called Roger.
However, it could have been a tongue-in-cheek joke to make fun of this genre.
However, it could have been a tongue-in-cheek joke to make fun of this genre.
(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 wig.
At 1:06, when the two twins talk to each other about the paintings, one of them moves slightly to the back of the screen but the other one continues to stare at the very same direction, revealing that it is one actor playing the two characters.
Each time one of the electric mixers is 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.
At around 17'50", there is some water on Columbo's pants around the zipper, revealing that they took several shots of him dropping the soap.
When walking away from the bank vault, Columbo calls Mr. Paris "Mr. Parks".
Columbo's partner tells him about the type of foot print (no shoes, flat foot) but not the size of the print.
At mid-episode, Ms. Peck states that the television was off for 20 seconds. However, in the "reveal" scene at the end of the episode, Columbo states the TV was off for 15 seconds.