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.
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: 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: While using the phone in his cell, Dr. Lecktor asks an operator to dial a Maryland area code, but is then connected to the University of Chicago. (00:30:10)
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: Towards the last scene, Howell and Hauer share the same shotgun (Ryder to take pop shots at Jim in the jeep, Jim to ultimately fire three shots to kill Ryder). The shotgun used can hold at most seven or eight shells, not the 12 or more that are fired between Ryder and Jim starting from when Ryder escaped the bus.
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: 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.