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.
Revealing mistake: In the scene where Ronnie visits Brundlefly for the second time we see Brundlefly walking on the ceiling and wall, however his shirt doesn't remain consistent with gravity. This is especially obvious when he pulls his shirt up to show Ronnie his new tumor and then places it back down. The shirt appears to be being pulled up to towards the sky from Ronnie's perspective. (01:07:30)Matt Rengers
Seth Brundle: I think you're making a mistake. I think you really want to talk to me.
Ronnie: Sorry, I have three other interviews to do before this party's over.
Seth Brundle: Yeah, but they're not working on something that'll change the world as we know it.
Ronnie: They say they are.
Seth Brundle: Yeah, but they're lying. I'm not.
Seth Brundle: I was not pure. The teleporter insists on inner pure. I was not pure.
Ronnie: I don't know what you mean.
Seth Brundle: A fly... got into the... transmitter pod with me that first time, when I was alone. The computer... got confused - there weren't supposed to be two separate genetic patterns - and it decided to... uhh... splice us together. It mated us, me and the fly. We hadn't even been properly introduced.
Question: Why exactly does Brundle experience a feeling of euphoria and strength after his teleportation? Why doesn't he turn into the Brundlefly immediately?