Revealing mistake: In the visions of Nefertiri scene where Nefertiri is fighting Anck-su-Namun back in ancient Egypt, Evie (Nefertiri) gets knocked to the floor and when she lifts her mask up you can see on her right index finger is a band aid/plaster. Did they have plasters back in ancient times?
Revealing mistake: After Rick and Evie find the bracelet the water starts coming after them. There are then shots of the water and then them running. Finally, they stop and hold each other. Shortly after that the wave comes over them. If you notice right before it hits them, it is their doubles you see. Not the actors.
Revealing mistake: In the scene where Rick and Evie break through the wall together with the water (after their son made the columns fall), you can see that Evie is lying on a skateboard on which she's being pulled forward. (It's a black one, visible when she leans over to Rick.) You can also easily see it under Rick.
Revealing mistake: The Anubis Warriors do not leave footsteps in the sand, but they do kick it up when they run. This is especially obvious when the army turns to dust at the end of the movie, leaving a pristine, footprintless desert. One could argue that the dust from the disintegrating Warriors covers the footsteps, but the dust also engulfs the Medjai, whose footsteps are still visible when they cheer.
Revealing mistake: During the fight against the Anubis warriors, in one shot you can see two men stab one of them in the chest. The warrior explodes and when the sand clears only one man remains. This is because all of the men were photographed separately and digitally composited together, so one of the computer artists made a mistake and accidentally cut out a character half-way through the shot.
Revealing mistake: When the Nile explodes through the corridor in the beginning, as the villain's goon is running by, you can see the wall suddenly get a little bit lighter in a circular fade pattern. This reveals that two separate images were used, and they were composited together. One day the goon was filmed, then another, the water bursting through was filmed, and the lighting was slightly off.
Revealing mistake: When Rick realises his destiny to kill the Scorpion King with the spear he backs into the wall with the painting on of a figure with the tattoo on his wrist. We see this is just a big prop because it wobbles as he backs into it, instead of just standing firm like a rock wall should.
Revealing mistake: During the very first shot of the scene where the heroes arrive in the canyon (during the "Wall of Water" scene), pay attention to the shot carefully as the camera tilts down, as several elements seem out-of-place. When Imhotep steps into frame, the lighting on him doesn't quite match the scenery around him (indicating he was shot against a blue or green screen and added into the shot), and the waterfall in the background moves somewhat unnaturally (slightly jittery if you look closely) towards the end of the shot.