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.
Factual error: When True Heart skydives down to Earth, she is wearing goggles with a wraparound strap. She takes these off and drops them in her backpack where Baby Swift Heart is hiding. When he pops up a split second later wearing the goggles, the strap is under his bonnet.
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 clan people look nothing like Neanderthals, they look like Cro-Magnon with a little dirt on.
Factual error: Charlie's 58 Impala is a sweet ride, but his license plates don't have the requisite year stickers. His car has the '56 series of plates, and each subsequent year would require that current year's sticker.
Factual error: Tuba pranks the store manager while he is reading adult magazines. One of them though (the one he is holding) is the August 1985 issue with Angela Nicholas on the cover. The previous scene took place at the Hong Kong Marathon of 1985, in January.
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.
Phaneron ★
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.