Deliberate mistake: "Long Live the King": The Seaview's 8 huge picture windows were a concession to design over practicality. Though they knew that such big windows could never withstand deep sea pressures, the producers wanted the characters - and the audience - to be able to see all the underwater wonders "up close." The "impossible" windows became one of the show's most popular features.
Factual error: "The Phantom Strikes": The Seaview's military history computers need an upgrade. The derelict German sub is identified as a WWI ship, the U444, supposedly sunk in 1918. There were only 167 U-boats in WWI. U444 wasn't built until 1942. And the sub's design is definitely WWII, not WWI.
Plot hole: "The Phantom Strikes": The Seaview's crew have a wild shootout aboard ship, and their bullets are seen to go all the way through a 6-inch-thick hatch door. Gee, you'd think that somewhere during their military training, somebody would have taught these guys why firing high velocity weapons on a submerged submarine is a really bad idea.
Factual error: "The Phantom Strikes": The derelict sub is said to have suddenly moved itself more than 30,000 sea miles from its original position. This makes no sense. The entire circumference of the Earth is only 22,000 sea miles. And if the sub had somehow circled the globe plus an additional 8,000 sea miles, there'd be no way to tell that it hadn't simply moved just 8,000 miles.
Continuity mistake: "Escape from Venice": Nelson takes the captured assassin's knife and says that Alicia was killed with one just like it. But he can't possibly know that. He never saw the body or the weapon. Only Crane was with Alicia when she was murdered, and Crane, still missing in action, hasn't been in contact with Nelson.
Continuity mistake: "The Cyborg": At the end, Crane shoots the cyborg Nelson in the forehead. It clutches at its completely intact face and falls down. We cut to crew reactions and then back to the fallen cyborg - and now its head is nothing but a jumble of circuits. Were those delayed-action bullets in Crane's gun?
Deliberate mistake: "Turn Back the Clock": Several of the roaming dinosaur segments here are repeated 2-3 times over. They're all recycled footage from producer Irwin Allen's own 1960 remake of "The Lost World," and Allen must have wanted to get as much use out of the dino shots as possible.
Factual error: "Killers of the Deep": Due to the extensive use of borrowed footage from the WWII film "The Enemy Below," the 1970s crew of the renegade sub (who are not German and who wore civilian clothes in most shots) suddenly acquire 1940s German U-boat caps and uniforms.
Factual error: "No Way Out": Victor is crawling around through Seaview's incredibly roomy ventilation ducts, pursued by several crewmen. But the air conditioning that's fluttering little ribbons on all the vent covers isn't, for some reason, disturbing a hair on any of their heads.
Continuity mistake: "11 Days to Zero": A saboteur on an overhead scaffold hastily sprays an X on the top of Nelson's car with white paint. In later aerial shots, the X is composed of 2 perfectly straight, thick lines stretching from corner to corner of the car's roof. No way was that X made in a few seconds with a spray can.
Continuity mistake: Throughout seasons 2-4: When the series went to color in the second season, the Seaview was redesigned with only one set of observation windows. But stock footage of the old multiple windows continued to appear for the rest of the series, usually with the film dyed blue.
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Factual error: "Invaders": The ship's doctor claims that carbon 14 dating proves the man from the undersea capsule to be 20 million years old. Not likely. Carbon 14 testing can date once-living matter only within the last 70,000 years, and is generally considered accurate only within the last 50,000.
Deliberate mistake: "Return of the Phantom": Kroeger's ghost takes over Crane's body when the captain is injured, but still alive. In "The Phantom Strikes," Kroeger told Nelson his host had to be dead first. This "fact" needed to be altered in the sequel so that Crane could survive to be rescued at the end.