Continuity mistake: When the firefighters go to the house of the first backdraft "victim" you see that he has gone through the windshield of his car. But when we saw him opening his front door and getting hurled away from the house he didn't land anyway near the car.
Continuity mistake: When the fire truck swerves out of the way to keep from hitting a car, and tips over onto its side, we see it precariously balanced on two wheels. When they show a shot of the inside of the fire truck, we see that the fire truck is scraping against the ground. The side scraping against the ground is the driver's side, while the truck rolled onto the passenger side. Then in the next shot we see the fire truck still balanced on two wheels and then it tips and falls onto its side.
Other mistake: When the firefighter is fighting with the arsonist, the gas line is disconnected from the fireplace. Just before the dramatic explosion, the gas line ignites. This is incorrect because the fire would have started from the outlet to the gas (from ignition source to fuel source).
Visible crew/equipment: When the fire-fighter is wrestling with the arsonist in the house (the arsonist gets a electrical outlet outline burned into his shoulder blade), they get blown off the front porch onto the lawn. In the cinema you could see a blue crash pad for the stuntman to the extreme left of the shot, but in the widescreen video you can't really see it, so they could have edited it out.
Continuity mistake: At the chemical plant fire when Brian climbs the ladder to the roof the angle of the ladder changes. When he gets on the ladder and climbs it is very steep. But in the next shot, when he gets to the roof, the ladder angle has changed.
Revealing mistake: The fire engine often shows up on the scene without any hose in the hosebed (on back of the truck).
Continuity mistake: When Brian visits Stephen on his boat, he looks through Stephen's music collection. He puts his can down near the radio, but when he turns back to the camera the can is back in his hand.
Factual error: When Bull goes charging into any of the fires in the film, he never buttons his protective coat. Any firefighter that did that would suffer severe burns. Also, he doesn't wear an SCBA, he would have suffered smoke inhalation without one.
Continuity mistake: During the ladder truck crash scene the truck tips to one side & then when everyone crawls out the truck is on the other side.
Continuity mistake: During Brian's graduation drink the bartender hands him a beer whilst ridiculing him about his numerous failed career moves. When the beer is poured it is about two thirds froth but as it cuts to another scene there is only an inch or so. Sufficient time has not elapsed for the froth to die down as it only spans one sentence of the bartender's dialogue.
Factual error: In some scenes at the climatic fire-fight, you can see that barrels have stickers indicating what's in them. Talking to a Haz-Mat Firefighter, in at least 3 scenes, the labels clearly indicate something that would've fried our heroes in that extreme situation. One barrel that he saw would've exploded because the chemicals label (that he could see; he could be wrong) react to ordinary heat!! Also, since this was a chemical fire, there would've been procedures to "decontaminate" firefighters instead of simply stuffing them into an Ambulance. It's possible that some industrial chemicals could pose a serious risk far worse than any Biological Agent if spread.