When, at the end of the movie, Ethan sees Nyah, he does not
   have a scar on his right cheek anymore. When he then hugs her,
   the scar is back.

  In the car chase between Ethan and Nyah, Ethan makes an
   obvious point of trying to fasten his seat belt (though not
   succeeding), however the belt is off and on again numerous
   times throughout the chase.

  After the fight with Ambrose, when Hunt is looking at Nyah the
   blood on his cheek and upper lip changes.

  It should have been more difficult for Ambrose to fool the
   scientist into thinking he was Hunt, as he is a full 6 inches taller.

  During the car chase in Spain between Ethan and Nyah, the
   center lines on the road change from a single white line (Europe)
   to a double yellow line (USA) several times.

  When Nyah is riding the boat to Sean's compound, she does not
   have her long scarf the very first time the boat is shown. She
   does have the scarf in subsequent shots.

  When Ambrose and Ethan are facing each other on the
   motorcycles just before they jump off, Ambrose accelerates
   with his right hand while Ethan accelerates with his left hand.

  When Nyah walks away from Sean at the races (after taking the
   envelope) she swings her arms and has not got the envelope
   under it, but in the next shot you see her drop it down to her
   hand after Hugh says "wait."

  During the fight between Ethan and Sean, Sean cuts Ethan's
   face with a knife and it's bleeding. A little bit later it is not
   bloody and it looks as if the wound is closed, then open and
   bloody again.

  The first night that Nyah is in Ambrose's house she meets with
   'Ethan' outside house and they talk for a couple of minutes.
   If she has been with both men, she should notice that this
   'Ethan' was about 6 inches taller than the real one since Sean
   is the taller of the two men.

  At the end, when Tom Cruise throws the knife at Dougray
   Scott's feet, there is a quick succession of shots consisting of
   the two in a fistfight. In one of the quick shots, there is a
   distinct black shape at the bottom right side of the screen that
   belongs to neither Cruise or Scott. The shoulder of a crewman
   who got a little too close, perhaps?

  Throughout the film, Ambrose has relatively short hair (on the
   back of his neck). But at the horse-racing scene at the
   beginning of the film, you can see that his hair at the back of his
   neck is exceptionally longer than at any other point in the
   movie.