Factual error: In the street scenes of Washington, DC, the cabs are Yellow Cabs. DC does not have a yellow cab system, but rather "gypsy" cabs that come in all colors. There are actually very few yellow-colored taxi cabs. (00:21:30)
Continuity mistake: John M is driving to the Eastern electricity station and chatting. The view out of his window shows him driving past the same distant hill at least three times. Also the speed is faster out of the passenger's side than the driver's.
Plot hole: After McClane, agent Johnson, and Ferrell leave in the police car, Agent Johnson radios another agent to have DC police clear a path for them. Then on Gabriel's screen it says they have a voice match to Farrell. The problem is Farrell never said a word, so how would they have a voice match to him? Without the voice match they never should have found them.brianjr0412