When the trucks are responding to the fire during the snowstorm there is an overhead shot of the trucks going down the street. In that shot, you can see that Ladder 49 is a more modern Pierce tiller ladder but then it switches back to an older Seagrave tiller ladder.
After Jack saves the girl from the fire, another firefighter points out a bleeding wound on his head. When Jack walks into the church later that night, the wound has vanished completely.
When the large explosion occurs while Jack is in the warehouse, you see the floor cave in under his feet, immediately followed by Jack sliding in. In the next close-up scene, Jack is obviously on the slope struggling to stay on that floor and not fall into the newly created hole. In the next scene, you see the floor is again whole, then caves in, and once more you see Jack start to slide down the angled floor toward the hole.
During the bar scene on Jack and Linda's first date, the position of the lime slice and strawberry on Linda's glass changes on both the first and second drink several times throughout the scene.
When Jack and Linda are at the bar and the guys make Linda chug her drink, she is about to finish it, but in the next shot her glass is half full.
Captain Kennedy orders all exterior operations shut down to execute Jack's rescue. In real life, all operations would be continued and a Rapid Intervention Crew (R.I.C.) would have been activated to execute the firefighter rescue.
Throughout the movie, Captain Kennedy is running the rescue operation of Morrison. That task would have been left up to a Operation Section R.I.C. Division Leader, Command would have only been given updates through the Operation Section Chief.
When Chief Kennedy is talking to trapped Jack Morrison on the radio from outside the building, when showing Jack trapped in the burning building, the type of radio Chief Kennedy is using changes multiple times. Sometime Kennedy is talking on a portable radio with no mic attached to it, and sometimes he is talking on a portable radio with a speaker mic attached to it, and is holding the mic in his hand. However it is common for the BCFD to run on both a TAC channel and the communications channel therefore causing a chief to have 2 radios.
During the wedding scene, Jack and Linda leave the party with children. These are not actually their children, but rather family members' children.
In the warehouse with the second victim, Jack gives him his mask to breathe through. When you pull off your mask, you should push the blue button. If you don't, the tube the air goes trough doesn't lock and all of your air goes away. When you put the mask on, and start to breathe, the tube unlocks itself, so Jack should have locked it again when he took the mask back.
The head nurse who talks with Jack is seen in the glass walking up behind him waiting on her cue as he looks into Tommy Drake's room.
When Jack is on the rope holding the man at the city high-rise fire, you hear a news chopper saying that it is over the fire scene, but after Jack says that he will drop the man if he doesn't calm down, you can see the word "POLICE" on the bottom of the chopper.
Much of the equipment (Big Ed flashlight, style of air-pack mask, etc) carried by Morrison (and other firefighters) in his "early years" as a firefighter was not available until more recent times (mid-'90s)
After Jack falls and is lying on the ground, P.A.S.S. device (the yellow device with the flashing lights on his belt) is sounding and you can hear dripping water in the background. In real life the pass device is so loud, it would be the only sound you could hear if you were next to it.
Ladder 49 clearly has a Federal Q siren in the direct center of the truck. Also it is clear that Engine 33 does not have a Federal Q. In multiple scenes of the movie Engine 33 is arriving on scene and you hear a mechanical siren coming from the truck, such as the apartment fire during the snow. Also even when you see Ladder 49 coming on scene, the mechanical tone sounds nothing like a Federal Q.
A the wedding, the Priest announces "Mr. and Mrs. Morris" rather than Morrison.
When Jack is arguing with Linda after he saved the man on the roof top, Jacinda Barrett's Australian accent is slightly noticeable.