Columbo leaves Helen on the boat for an "overnighter" and goes straight to the diner for chili. From the diner payphone he calls "Mrs. Stewart" at home and she answers. She would have still been on the boat at this time.
Close-ups of General Hollister showing his team medal shows Columbo with hands clasped together, but when scene changes to a two-shot, his left hand is in his pants pocket.
During the test drive, the blue cushions on the yacht's foredeck shift between being uncovered and covered up with a white canvas several times.
The equally blue cushions on the aft deck are uncovered when the yacht leaves the harbor, but seconds later, and for the rest of the test drive, there is a white canvas cover on top of them.
Columbo's lit cigar disappears as he crosses the threshold from General Hollister's bedroom to the living room.
When Helen says "Now that is the simple truth," Columbo takes his hand off the llama's rump and folds his arms, but when the scene cuts to Columbo alone, his hand is back on the rump. Cutting back and forth, Columbo's hand remains on the rump in his shots while his arms are folded in Helen's shots.
The ribbons on the general's spare uniform are not correct. What is shown are two identical sets of ribbons, stacked on top of each other to make it seem like more ribbons were awarded. Since medals are worn in a very specific order, this is a mistake.
When the General shows Columbo his memorabilia, he refers to the ammunition magazine as a clip. No one in the military, especially a General, would ever call an ammunition magazine a clip.
The German swastika at the war display is backwards. This was a practice of Hollywood for quite a time in order to cater to sensibilities.
The portrait of Maj. Gen. Hollister has an incorrect number of stars. His uniform is shown and has the correct number, two, but the painting shows four.
During the test drive of the yacht you can see the wake of the chase camera boat.
Many of the books on the general's bookshelf are duplicates. Besides the Mrs. Melville title (in white - 2 next to each other), "Challenge" (in blue - at least 3, one turned upside down), "High Season" (in gold - at least 2, one flipped), at least 2 of a title in silver, and at least 3 of a 2-word title in red. While the general did maintain a number of things "for show", it seems unlikely that he would purchase numerous copies of the same titles.
When Columbo calls Mrs. Stewart on the payphone in the diner, he dials just six numbers.
General Hollister says he is sending his memorabilia to his alma mater, Marine Military Institute, MMI. This real-life boys' military academy was founded in 1965. Even if the general were part of the first class and a senior at the time (approximately 17 or 18), he is too old in 1971 (episode air date) to have gone there only 6 years before.
When the general is testing the engines on his yacht, the wake of the camera boat can be seen in foreground of the long shots.
During a conversation between Columbo and Mrs. Walters, a microphone is visible at the ceiling.
When Lt. Columbo talks with Mrs. Walters in her apartment just before Helen Stewart comes back a microphone is visible above them.
The General kills his victim instantly with one close-in shot. So he was either shot in the head or in the heart. Either wound would produce copious amounts of blood everywhere. Yet when Columbo enters the house, there are no bloodstains to be seen anywhere. It would be impossible to clean all such traces by that time.
It would be impossible to witness the shooting inside a house from the distance that the witness was from the house, especially during the day since the interior of the house would be much too dark to see anything.
Columbo arrives only a few hours after the shooting. If someone had fired a gun inside a living room the smell of gun powder would still be immediately noticeable, especially by a detective who'd be looking for it. Also, they did no forensic evidence collection beyond asking questions. A fired gun would leave fallout traces on the carpet. Police were capable of this even in 1971.
At 10:40, the policeman (Sanchez) reports to his superior on the radio and names the possible shooter which lacks discretion, especially given the fact that he very well knows about the General's past and fame.