Revealing mistake: In the big speedboat scene near the end, when Cage is hanging off the side of the boat the stunt double is obvious in wide shots, where the entire boat is visible from above. He has big long sideburns, which are in stark contrast to Cage's short hair, visible in closer shots. At various other points in the movie, stunt doubles appear of much different physical builds than the two actors, with markedly different hair colours, and I seem to recall that at least one of them had facial hair.
Revealing mistake: At the boat scene when Archer is holding on to the red boat and is skiing on the side, you can clearly see the waterproof boots he is wearing.
Revealing mistake: In the speedboat crash where the film is slowed down, there is a medium shot of Troy and Archer flying through the air. It is very obvious that neither are the true actors.
Revealing mistake: In the scene when Castor Troy kills Agent Winters the sky is blue. There is only one cloud just above the fuselage. This cloud doesn't move, at least not that fast as it should as the jet is near take-off speed. The aircraft stands still when the actress falls out of the jet. In the ensuing shot, when the dummy falls out, the sky is overcast and the aircraft moving again.
Revealing mistake: When Castor is pushed into the blowing heat from the plane engine by Sean and is thrown back, the man that lands against the grill is so obviously the stunt double; he has a different hair cut than Castor and really doesn't look anything like him.
Revealing mistake: At Erehwon, Sean (as Castor Troy) is given steel magnetic boots to wear, for the prison's important security feature. There is a close-up of these heavy duty boots during his 'exercise'. Then, when he is taken to the clinic there is another close-up, and there are a number of changes to the boots style. More importantly, one of the side "bolts" has fallen off, revealing that it was merely glued on the prop boots.
Revealing mistake: Just as the red speedboat heads toward shore, before it crashes, the white and red truck cranes are on the dock and the white crane arm is angled backwards over the dock. In the shot facing the speedboat, as it explodes, that crane arm is now angled the opposite way, over the water, since it was used in the stunt.