Near the end, a beer keg rolls by one female character three times between shots.
When the creatures are attacking the townspeople while celebrating at the Salmon Festival (about 1h 05m into the film), you can hear the screams are being played on a loop at approximately 10 second intervals.
When the Salmon Queen bashes the skull of one of the sea mutants right after her bikini top is yanked off, she ends up fully topless, but in one shot, her 'beauty pageant' sash and her bikini top strap can be seen on her back.
When Peggy and Jerry are first attacked by the Humanoids, Peggy has a green design on one breast-cup of her bikini. Throughout the scene that design changes back and forth from one cup to the other.
When Linda is first shown driving her pickup down the road for help the windshield appears to have a huge hole in the center of it. In the next shot, the windshield is intact.
The first attack scene on the boat when the winch engine is heard running then runs out of fuel during the conversation with the boy there is no spark plug wire going to the engine's spark plug, so being out of fuel is the least of the problems.
Mispronunciation of the fish name. Coelacanth is pronounced see-la-canth not co-e- lacanth.
Once the humanoids break through the bottom of the dock, the commotion starts. And the first eight or nine seconds of screaming is "looped" and continually heard throughout the rest of the attack at the carnival.
In the first boat sequence, the winch is supposedly under a lot of strain but all of the lines are loose. They would be tight if the winch motor was working to attempt to pull in the net.
When Jackie is pulled underwater by a humanoid, you can see that the film was merely run in reverse, (hence the water rolling UP his head) from the first time he emerged after falling in.
When the humanoid is lying in the road after he fell off the truck's hood, he presumably starts to roll over. But it's just recycled film in reverse from a few seconds earlier.
When seen from the outside, Linda's pickup truck clearly has a large center section of the windshield cut out for the camera to do interior shots of Linda inside the truck.
As a creature looks into a woman's bathroom window, she comes to open the door. No bathroom should have a door to the outside in a regular house.
Dr. Blake incorrectly pronounces Ceolacanth as "cola-canth", not "seela-canth".