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.
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.
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.