In the second scuba scene (the one with the two divers in the diving bell), one of the diver's gear is different when in the diving bell as compared to when he's seen underwater. His wetsuit is different (look at the orange trim on the shoulders carefully). Also in the diving bell, he has just a regular single hose regulator, whereas underwater, his tank also has a double hose regulator setup attached to his tank (though he's still using a single hose regulator for breathing).
When Bo Hopkins is seen suiting up before the final dive scene towards the end of the film, he is wearing a different wetsuit than his stunt double underwater. The diver underwater is wearing a suit with orange stripes on the arms of the suit, but topside, Bo's suit doesn't.
Mike is only wearing shorts throughout the scene where he's feeding the whale, except one jarring close-up of his arm throwing food into the whale's mouth, where he's wearing a white t-shirt.
Although the title of the film is "Tentacoli" ("Tentacles"), and it is about a murderous giant octopus, octopuses do not have tentacles, they have arms. Scientifically, the definition of "tentacle" is commonly accepted as an elongated structure with suckers at the very tip. A squid normally has two tentacles, along with many arms, but octopuses do not have tentacles.
Octopuses don't roar.
Claude Aiken wears a sheriff's badge but has on a blue jacket. Sheriff deputies wear green jackets to match their pants. In a scene by a life guard station he is standing next to a police car with a single blue light. None of the police agencies in San Diego county use only blue lights.
The police dispatcher heard on the radio under the car's dash mentions the corner of Balboa and Clairemont drive. This intersection is in San Diego PD's northern division, not the sheriff's office.
There is a scene where a character on a small boat witnesses the drowning of a man after he is grabbed by the octopus while swimming. She sees a large amount of air bubbles rise to the surface of the water where the man disappeared, but a few feet away there is a second, smaller patch of air bubbles, as if there are two people under the water, revealing underwater crew members.
The opening scene is along highway 101 in north county, the next scene where the baby is taken is on harbor island just south of the airport in San Diego. There are no homes in that area so there would be no reason for a school bus to drive onto the island.
When they are discussing what to do about the giant octopus, Turner incorrectly refers to the beast as a giant squid.