When the helicopter carrying Carter lands, he gets into a police car, which drives off with the Golden Gate Bridge in full view in the background. This is right after the octopus has begun pulling itself up on the bridge's south tower, yet in the shot the octopus is plainly not on the bridge. Soon after, Carter drives that same police car out onto the bridge, but when he arrives in mid-span, the car is a different make, model and year, matching the (miniature) police car subsequently crushed by the octopus's tentacle.
During the airplane search for survivors after the first monster attack, the pilot sights two men in a rubber raft in the ocean, but when the scene transitions to the naval hospital, four rescued sailors are brought in.
When Professor Joyce calls John Carter from operations, Professor Carter tells her to take the reporters to the pier to show them the weapon, but Professor Joyce never mentioned that there were reporters in the room and he certainly couldn't see them through the radio.
When the submarine is caught by the creature, Commander Mathews orders All Back Full (reverse), an order verified by the exec a few minutes later, but when the sub is released, it immediately moves forward, not back.
Near the end of the film, when fear of the giant octopus is at its most frenzied, there is a highway scene purportedly showing people fleeing San Francisco in their cars. However, this shot actually shows traffic coming INTO the city. The shot is of I-80 west and the cars are just entering San Francisco from the San Francisco-Oakland Bay Bridge. It is evident that the traffic is flowing into San Francisco and not away from it, because one of the bridge's two towers can clearly be seen in the background perpendicular to the road. The "Bay Bridge" (as it appeared in 1955 and still appears today - a new bridge is currently under construction) consists of two spans connecting San Francisco and Oakland via a small island in the middle of San Francisco Bay. The western span connecting the island to San Francisco is a "double-deck" suspension bridge with traffic flowing into San Francisco on the top deck and traffic flowing out of the city on the bottom deck NOW, and for the last half century, but at the time of the movie, the upper deck had lanes of car traffic in both directions, whilst the lower deck had train tracks and bi-directional truck traffic. Cars were not "banned" from the lower deck, and traffic on the lower deck (both directions) was lighter than the upper deck. Eventually, in the 1960s, the train tracks were removed, and each deck was made one-way.
When the authorities are detonating undersea mines to discourage the giant octopus, explosions are shown in S.F. Bay. Those explosions are actually taking place on the surface of the water, since the surface shock-wave is clearly visible and no water spout is created.
When Commander Mathews and Professor Joyce reach the beach at Astoria, Oregon, a sign welcomes them to the 11th Naval District. Oregon is in the 13th Naval District. The 11th District is in Southern California.
Faith Domergue says toward the end of the movie that another giant octopus attacked in the 12th Century as a result of the eruption of Mt. Vesuvius. The AD 79 eruption of Vesuvius is the most famous, but in the 12th century it erupted in both 1139 and 1150.
In all the shots of the monster sprawled across the Embarcadero, cars can be seen driving normally along the street, even right under its tentacles. It is highly unlikely that motorists would continue to calmly drive past a giant octopus attempting to come ashore, let alone beneath its tentacles.
When the octopus's tentacle is smashing the entrance to the ferry terminal, people are seen running away along the sidewalk on the right side of the screen. In a close-up of the fleeing people, the last stragglers, including several policemen, are shown running away. When the shot cuts back to the full scene, again showing the tentacle, there are suddenly many more people running along the sidewalk, and those stragglers appear once again at the very end of the scene.