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.
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. If this were a shot of cars traveling out of the city (with the camera placed at a similar angle showing the on-coming traffic), it would be impossible to see either of the towers because they would be obstructed by the top deck.
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.
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.