Audio problem: During the Christmas season (December) Travolta and Phoenix discuss a transfer to an aide's position. That same night Phoenix discusses the possibility with his wife - with crickets chirping away in the background. Not in Baltimore in December.
Continuity mistake: In the scene where Jack and Lenny are arguing about Dennis's death, the cup on the table gets knocked over. The next shot, the cup is standing up. The next shot, it's down again.
Continuity mistake: In the scene at the abandoned building where the firefighter falls through the roof, when they rip the plywood off the door, you can see the whole piece of wood fall. But when the shot goes back to Jack as he opens the hose line, the plywood looks like it snapped in half and is still in the doorway.
Continuity mistake: In the scene where Jack rescues the worker from the warehouse, while lowering the victim - the rope is wrapped around the beam - but in the overhead shot as the floor gives way - the rope is only laying across the beam.
Continuity mistake: In the rope rescue scene, when they first arrive, the snorkel is nosed into the tiller truck right below the action. Then it disappears, then reappears in one of the final shots of that scene.
Continuity mistake: In the scene where the floor collapses at the grain facility, Jack's helmet comes off as he is falling, but in the next shot it appears back on his head for a split second as he continues to fall.
Continuity mistake: When Jack is lowering the worker out of the warehouse, the number of carabiners on the rope goes from one to two then back to one in the various shots.
Continuity mistake: During the response to the apartment fire in the snow, one of the responding shots is of a new pumper and a fully enclosed new tiller truck, instead of the actual units which are quite a bit older.
Deliberate mistake: The nose pieces on the face masks have been removed from all of the SCBAs used in the movie. The nose pieces are essential to keep the mask from fogging up, but were removed to allow the audience to see the all of the actors' faces.
Factual error: Before Jack starts to rappel down the side of the building to save the man, he tightens his carabiners as tight as they go. Anyone who has undergone training (assuming that a fire fighter was properly trained) would know to twist the carabiner nob a half turn back, otherwise after climbing the nob would be so tight that it would literally take the "Jaws of Life" to get it open.
Continuity mistake: When John Travolta and his men are trying to enter the burning house after the troubled firefighter, he goes from having gloves on to not then back to wearing gloves.