Revealing mistake: In the scene where Will Smith first jumps aboard the rear of Loveless's train, as he climbs onto the roof of it you can see contrails from a jet aeroplane in the sky (top left).
Revealing mistake: It's painfully obvious that CGI backgrounds in panorama shots are fake. A good example is just after they get their magnetic collars off and are walking through the desert.
Revealing mistake: When Loveless removes the pin connecting the cart to the horses with the nitro in crates, it begins to roll down the cliff. There are already tracks for the cart to follow as it goes off the edge.
Revealing mistake: After removing their magnetic collars, Gordon gets dragged across the floor. Check the sand and you'll notice a trail ahead of him from previous shots.
Revealing mistake: When the saw blade thrower begins powering up after Jim West steps out of the magnetic boundary, the gears beneath the saw blade rise and begin to spin, which are meant to engage the saw blades' rotation, but in a single frame, these gears are shown instantaneously back in their resting places, as the saw spins on its own freely before being flung from the machine. (01:02:20)
Answer: Assuming it's the same technique used for Gary Sinse in Forrest Gump, Mr. Branagh was probably wearing blue leggings which was used to digitize out his legs.
Lisa T Ellis