Revealing mistake: When the Bronco is underwater at the end of the movie, Claire's hands are tied to the steering wheel. Doug rips the steering wheel off. Anyone who has worked on cars knows you need a puller to get a steering wheel off. Also, the steering wheel is gone when Claire swims to the surface.
Audio problem: When Doug is staring at the terrorist and says, "I can see him, he's right in front of me", you'll notice that his mouth moves a little before words are actually said. This is because he first says "Jesus", but they took it out because the character playing the terrorist is Jim Caviziel, who played Jesus in Passion of the Christ. The filmmakers did not want to get a laugh out of that line which it inevitably did in test screenings.
Factual error: During Claire's post mortem, Doug takes a picture of her face. Her head was tilted towards the camera as were her eyes. When the picture comes out, it shows that she was faced away from the camera.
Continuity mistake: When Carlin goes to the dock he gets out of the Hummer carrying a gun. The gun changes hands between shots as he is walking.
Factual error: In the Hummer scene, the signs that read "I-310 South, Bayou Bouef" and "U.S. 90 East, Bayou Bouef" in reality read "I-10 West, Baton Rouge" and "U.S. 90 East, Claiborne Ave".
Continuity mistake: When Denzel Washington is taking off his clothes to be sent back in time. We see him take off his blue shirt. The shot then changes to look at the other guy and we see him take off his shirt again.
Continuity mistake: When Clair is on the post mortem table being examined, look at her eyes. When her head is turned to the right her eyes are as well but when they turn her head to the left her eyes are left as well. Another scene shows her eyes appearing to be looking straight up. Unless her eyes were manually moved they wouldn't change positions after death.
Continuity mistake: When Denzel first leaves in the hummer he is on the Westbank heading toward the CCC Bridge to the Eastbank (downtown New Orleans). Once he makes it to the bridge, he's suddenly headed back to the Westbank (away from downtown) but on the Eastbank bound span (bridges are one-way), only to exit the freeway and suddenly appear on the Eastbank and cause the accident with the 18-wheeler.
Continuity mistake: In the scene where Doug and the terrorist are talking about 'destiny', the terrorist has a tiny bit of his cigarette left. But when the camera quickly shows a shot of Doug's face and then comes back to the terrorist, his hands are almost in exactly the same place, but the cigarette is gone.
Deliberate mistake: When Denzel Washington wakes up in the hospital he unplugs the cables attached to the heart monitor. By doing this he shuts down the monitor. Actually those monitors go into yellow alert by disconnecting the heartbeat sensors. They can be configured to do nothing at all by disconnecting but they will never shutdown this way.
Deliberate mistake: After Doug finds where Minuti was killed and set on fire, he walks over to a dock where he sees Minuti's arm being eaten by alligators. Minuti had been dumped in the water 4 days prior, yet his hand is perfectly intact with no signs of burning, bite marks or weather/water damage of any kind.
Continuity mistake: When the red Ford Bronco is driven off the ferry (ignoring all the impossible factors required for it to successfully do so), it flips through the air and contacts the water going backwards, its right/rear tail light entering the water first. The closer next shot shows the truck sink somewhat as it continues to flip backward, onto its roof. The very next shot shows the interior of the truck as it enters the water right-side up and going forward as water rushes in. Denzel lurches forward and down, driving his face into the water that is about waist level. The next exterior shot is underwater as the truck now lands in the water, nose first and right-side up. It then begins the expected slow dive, nose-first.
Other mistake: When Mutini first gets shot, the gunfire supposedly comes from inside the vehicle, but the windows and doors are closed, the glass doesn't shatter when the shots are fired. There are no visible bullet holes when the door is opened.