Deliberate mistake: When Harrison Ford and Mary Lynn Rajskub are crossing the tracks, its Rajskub who's driving the car, not Ford. This is because the filmmakers originally had Rajskub driving the entire way. They later refilmed the drive with Ford driving, but kept the shot of them crossing the tracks the same. Ford and director Richard Loncraine confirm this on the special features of the DVD.
Firewall (2006)
1 deliberate mistake - chronological order
Directed by: Richard Loncraine
Starring: Harrison Ford, Paul Bettany, Virginia Madsen, Jimmy Bennett, Carly Schroeder
Factual error: Text doesn't actually move up a screen, it only appears to; it's really only blinking lights. As such, the improvised scanner wouldn't work, you'd only get a small slice of each line.
Bill Cox: You designed the software, you find me a way in.
Jack Stanfield: I wouldn't know where to start.