The 3 women are driving a 1958 Ford Fairlane. When they get a flat tire the shot of the tire going down shows clearly a wheel on a 1959 Chevy.
Dr. Gavin says "when Tina was killed that night." Tina was killed in the daytime during the beach party.
When Elaine goes down to the water at Fingal's Quarry she is carrying only a box that contains the Geiger counter. When she runs away she is carrying the Geiger counter (without the box) and an attaché-type briefcase.
When Elaine gets out of the car at Fingal's Quarry she is carrying a loose Geiger counter and an attaché-type briefcase. When she approaches the water she is carrying a larger box from which she removes the Geiger counter. When she runs away she is again carrying the Geiger counter (without the box) and the briefcase.
At 31:25 the bucket placed over the door in an earlier scene has disappeared when the door is opened.
Pure sodium is a highly reactive metal. It is kept stored in oil or gasoline (not loose in tubs, as portrayed in the movie), as the moisture in air is enough to trigger a violent exothermic reaction.
A detective says he's going to run a "carbon 14 test" on scales found at the first murder scene and, something to the effect of, "there's been major breakthroughs in the world of genetics." Carbon 14 is used for carbon dating, not genetics and it would be incredibly useless in a murder investigation.
Fresh water tropical fish portrayed as salt water ocean fish.
Whenever the band plays, their voices have a reverb yet they have no microphones and none of the guitars are plugged in.
When the traveling girls get attacked by monsters in the woods, and one of the monsters throws one of the girls in the trunk, the monster's suit gets caught in the trunk when he closes it, and has trouble getting out.
Many examples of speaking, screaming, and laughing while characters mouths aren't moving, or are moving but mismatched with the audio.
As the monsters emerge from the water to pursue the heroine you can very clearly see that the second monster has a human head sticking out of its neck.
When Hank goes to Manhattan, he's seen driving up (in a northerly direction) the FDR Drive. If he was coming from Connecticut (where this movie is set), it would be more likely for him to drive down (in a southerly direction) the west side highway. Moreover, he's seen in Times Square, Greenwich Village, and the west side near the George Washington Bridge. To anyone at all familiar with even modern day NYC, it appears that Hank is driving around and around in circles.
In the lab, the doctor examines the severed hand of the monster. He claims that its tissue resembles a sea urchin's, which is a form of protozoa. A sea urchin is not a protozoa.