In the "steak experiment" sequence, Seth cuts the meat in two pieces, and teleports the bigger one. Later, Veronica eats a bit of this bigger half and finds that "it tastes like a steak", and when Seth gives her a piece of the smaller one and says "now try this teleported half", she finds that "it tastes synthetic". This is a continuity mistake. (It has been suggested that Seth deliberately misled Veronica as to which steak was teleported, to test Veronica's psychological reaction. However, this is unlikely; if the test were merely to determine Veronica's aversion to eating teleported meat, then it would not have led directly to the realization that the computer was incorrectly reassembling flesh).
After Veronica tries the piece of teleported steak, she's seen drinking from a glass of water that wasn't on the table in the establishing shots. In the deleted scenes for the film, the shot is part of an extended deleted moment where Seth pours her a glass of water and hands it to her.
When the first baboon arrives in Telepod #2 he hits against the glass door of the pod, leaving a bloody mark. In a subsequent shot the amount of blood has significantly increased and the shape of the mark is different too.
When Brundle is sat at his computer, he bites the pencil and his teeth fall out onto the keyboard making a mess. A later shot of the keyboard reveals no teeth or blood on the keyboard.
When Seth jumps out of the final telepod at the very end, Ronnie starts to look up to see what has happened. In her close-ups the image has been reversed.
When Brundlefly dissolves Stathis's hand, a considerable amount of vomit falls on his chest, but there is no sign of any damage unlike his hand and his foot.
When Veronica is playing the cassette tape of her recorded conversation with Brundel for her former boyfriend, the direction of the tape and the amount of tape left suggests that it just began from the beginning. However, the portion of the conversation that is being played is just before her tape recorder begins beeping, indicating that the tape is at the end and needs to be flipped over.
In the last scene when Veronica shoots the shotgun, in real life it would have ripped away from her, and probably injured her hands and wrists. You must brace a shotgun to the body to prevent injury from the recoil.
At about the 25 minute mark Seth corrects Veronica when she calls the baboon a monkey. But baboons are monkeys.
The movie is supposedly set in the USA. The person at the bar mentions that Seth eats "chocolate bars." This is how a Canadian would describe them. The correct term in America is "candy bars."
"Every time Seth or any other creature for that matter goes through the teleporter, they're completely naked so as to avoid confusing the computer and introducing more foreign materials. However, at the end of the movie, Brundlefly shoves Veronica into the telepod fully clothed. Whilst the resulting merge of DNA would be human, it would in fact create a worse mutation due to there now being a combination of human, fly and clothing material merged." ... However, at this point in the film, Brundlefly was decomposing at an accelerated rate, barely able to communicate, and certainly not having rational ideas. He wanted to fuse himself, his love, and his unborn child into a single fused being. He obviously wasn't thinking things through when he forcibly shoved her in the pod and began this process just moments after he had the initial idea to do so.
Early on, to move his experiment along, he transports a steak as a experiment, and it's on a plate. Later on in the film, he accidentally merges with a fly as he has not programmed the computer to separate objects. The steak and plate do not merged because the teleporter treats inanimate objects differently.
When Seth is changing into a fly, he demonstrates how he can walk on the ceiling and walls like a fly. While upside down on the wall, he lifts his t-shirt to show the side of his body then places it back down. The reaction of the t-shirt to gravity (or lack thereof) made it obvious that he was filmed right side up and then the film rotated to get the effect.
Seth (or rather, Jeff Goldblum) is not naked when he goes into the telepod. An elastic band can be seen around his waist, indicating that he is wearing a "dancer's belt" meant to conceal the genitals.
When Seth types into his computer, there is a sound of him hitting keys. When the computer replies, it is preceded by a triangle shaped prompt, and accompanied by a buzzing sound. When describing "The Brundlefly Project," the "Solution" is preceded by the prompt symbol, but rather than the buzzing sound, the sound is of Seth typing, making it unclear whose idea this really is.
The camera crew is reflected in the glass of the telepod.
Just as BrundleFly and the part of the Telepod are teleported together, a piece of an arm of a puppeteer in a blue shirt can be briefly seen behind the Telepod.
When Brundlefly opens the medicine cabinet to deposit his teeth, the mirror reveals a person in a red long-sleeved shirt pointing at him.
When Veronica returns home to her apartment to unexpectedly find Stathis in her shower, and the camera begins to track and follow her down the hallway to the bathroom, the camera equipment and crew are reflected in the glass panes of the French doors.
When Veronica returns to Seth's lab immediately after he has experimented on himself, a crew members hand can be seen reaching out and touching the door frame as the camera follows her into Seth's bedroom.
The common housefly has a chromosome number of 12, humans have 46. The combined organism Brundlefly would not have lived long or lived at all. Organisms that have mismatched chromosomes are stillborn or miscarried. Compounding errors during cell division results in death.
Seth Brundle merges with a fly since a fly had flown into the telepod before transition. This would also mean that Brundle would merge with any other living object in the telepod including any bacteria living in his body and any virus currently in his body.
Additionally it has to be assumed that the baboon that transported successfully also had fleas on his body so the baboon also should be transformed into a baboon-flea-bacteria-mixture, not just a baboon.