Plot hole: The whole problem with the teleporter occurs when fly DNA is mixed in with Seth Brundle's DNA, starting his transformation into the Brundlefly. Brundle is on a hiding to nothing from the word go, and the fly is irrelevant. Humans are a walking talking mass of foreign DNA - we are host to one trillion bacteria all of which has a complete compliment of DNA, as do various tiny mites that live in our hair follicles and all sorts of single cell organisms in our gut. If the transporter serves to mix the DNA of all living creatures which are in the transporter pod at the time Brundle would turn into a half-man, half-bacteria. Incidentally, DNA from a bacteria, an amoeba or a hair follicle mite would be just as 'compatible' with human DNA as that from an insect. It's quite a simple chemical, really.
Audio problem: A soda can that is supposed to be full hits the man. When the can hits the ground you can hear that it's empty.
Other mistake: In the final scene of the movie, Megan starts a boat motor and uses the propeller to hit Jason in the face, who has been chained down underwater. But if Jason can only barely reach her foot from underwater, there's no way the boat propeller is underwater far enough to reach his face. (01:17:50)
Factual error: Right before Frank humiliates Dorothy for the first time, he blows out one of the gaslights in her apartment. That is never done in real life, for the same reason you would not blow out the pilot in your stove.
Continuity mistake: In the sedan/pickup chase, both of Cobra's headlights are shot out at the start with pinpoint accuracy. A minute later as the cars race down the freeway, the holes where Cobra's headlights used to be are filled with black plastic discs.
Factual error: When the Delta Force lands on a beach, prior to their assault, their landing craft is lit up like a christmas tree. The commandos wear dark uniforms and have blackened their faces, but they also carry bright white ropes around their chests. Some of them are scuba-divers, wearing yellow oxygen-tanks. In short, in the real world they would have been spotted within minutes.
Revealing mistake: In the car chase scene where the cop is after Roland, you can see that there is no key in the ignition of the cop's car.
Continuity mistake: During Kevin Bacon's race with fellow messenger, the scenes shift back and forth from New York to San Francisco. In one city, he's on a fixed gear (no freewheel on back wheel) and when the race is taking place in the other city, he's riding a single-speed. Basically, the cityscape and bicycles switch from scene to scene.
Revealing mistake: It is obvious a dummy was used when Steve is thrown from his easy chair into the kitchen cabinet.
Continuity mistake: When Eddie and Michel are in the police station and Losado comes to pick up Michel, her hair clip is on the left side of her hair. After Losado and Michel get into the car, her hair clip moves to the right side.
Continuity mistake: When Jim and Brandon are knocked into the water, Brandon dies, but somehow manages to grab hold of Jim and pull himself up.
Factual error: In an early scene, the high school science teacher produces a schematic diagram of the two nuclear bomb designs created by the Manhattan Project. He says, "Here are two designs based on plutonium." Actually, only the Fat Man bomb uses plutonium. The other, the Little Boy design, uses uranium.