Factual error: There is no possible method of "fusing" the genetic material of a common housefly (Musca domestica) and a human. The housefly has twelve chromosomes, humans forty six. There is no way to combine the two in order to produce a viable organism. Thirty four of the human chromosomes would have no matching chromosome to "fuse" with, meaning the physical characteristics coded by those genes would not form. The Brundlefly would be missing three quarters of his human body.
Suggested correction: There's no possible way of teleporting physical objects either, but it happens in this movie. This is science fiction. These kinds of "factual errors" are not valid.
The film presents no scientific explanation for "teleportation" but does for "genetic merging." Teleportation is possible in this film's universe, but "genetic merging" is impossible in any universe.
Genetic merging is possible in this film's universe; that's the whole point. It doesn't matter if the explanation doesn't stack up, it still works.
Factual error: The mounted machine gun fires far too many shots for the limited supply of ammo it has in the box attached to it.
Factual error: The "whaling boat" is too small to function as such. It isn't large enough to hold a fin, let alone disassemble a humpback whale.
Factual error: When Steve Martin dies with the mask on, the inflation bulbs (the little black bags that inflate and deflate) deflate completely. Actually they should INFLATE completely and stay that way. The gas is still on full blast and Steve is no longer inhaling (which would cause them to deflate). It was done only to emphasize Steve Martin's demise but it's factually incorrect.
Factual error: When the first robot is trapped in the lift, the teenagers shoot the propane tanks on top of the lift in order to get them to catch fire and explode, killing the robot. However the teens are firing pistols or shotguns, and these do not contain incendiary ammunition, which is what is required to make the propane to catch fire.
Factual error: When we first see Kurgan in modern day New York, he begins to assemble his sword in a motel room. The very fact that he is assembling his sword the way he is, snapping segments together, especially the blade itself, which is segmented in the middle, would make that quite possibly one of the weakest and most inefficient weapons a person could ever wield. Being that it is segmented the way it is, it is structurally weak, and a clash against another sword, or any hard surface for that matter, would break the segments apart easily.
Factual error: The aircraft are definitely post-WWI. Biggles is flying a De Havilland Tiger Moth, which appeared in 1930s. The German fighter is probably the PT-23 Stearman.
Factual error: When the alien craft speeds away from the NASA base, its slipstream tears off the roof of a small booth. But the wake also should sweep the people in its flight path down to the ground; it is too close for that not to happen.
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.
Factual error: Mission control begins an auto-landing sequence for the shuttle, but mission control did not have an ability to remote pilot the shuttle when the movie was made and wouldn't get it until 2006.
Factual error: The marines' rifles are supposed to fire "standard armor-piercing rounds", which should have done way more damage than producing a few sparks on the frail sheet metal table in the med bay when Hudson shot the face-hugger threatening Newt. "Standard AP rounds" would have pulverized the table and created a huge hole in the wall behind it.
Factual error: When Number 5 is watching Saturday Night Fever, he is watching and grooving to the climactic dance-off scene in Saturday Night Fever on TV. Stephanie then enters and the scene on TV shifts to one much earlier in the movie he's supposedly watching. (00:57:15 - 00:59:30)