Revealing mistake: At the beginning of the movie we see Rick and Evy almost drowning in crystal clear water. Had that water really been from the Nile, it would have been muddier.
Revealing mistake: During the final battle, when the burning oil-pots are being knocked over, you can see the weapons on the sides are rubber from the way they bend and wobble.
Revealing mistake: In the opening battle, when the armies organize before charging, you can see some men walking right through each other, where digital doubles have been added into the scenes. Hard to catch, but it happens at least once or twice.
Continuity mistake: When we first enter the museum there is a long shot of the resurrection area with Evie at the top of the screen lying on a wooden stretcher. She is conscious and seen moving around to adjust the ropes. In the next shot, she is unconscious and is only woken once she is bumped on the table. (00:35:35)
Deliberate mistake: At the beginning, we see the Scorpion King leading his army into battle. However, despite people being stabbed with swords, and slashed, there is no blood on any of the soldiers, nor on their weapons. Kind of hard to believe war was that clean back then. (00:01:40)
Deliberate mistake: According to the DVD version, the Army of Anubis roars in ARABIC. Rather strange for a monstrous army of an ancient Egyptian god... (01:44:15)
Factual error: We get a panoramic view of London with the houses of Parliament, St Paul's Dome and Tower Bridge. The problem is that Tower Bridge has been moved next to the Houses of Parliament (they have put it in place of Westminster Bridge) and it should in fact be not visible at that point but several miles down river. St Paul's is also on the wrong side of the river. If this is where Tower Bridge is in the film then there are no low bridges for the bus to hit in the chase which follows the escape from the British Museum. Were it in the proper place there are quite a few. (00:20:35)
Factual error: Our Heroes are running after the Bad Guys who have kidnapped their "Precocious Kid." They are chasing them in a dirigible. We see them flying across the full Moon. A few nights later, we see them flying across... the full Moon. The phase of the Moon is probably the single most abused astronomy idea in movies (besides sounds in space). The Moon goes through phases as it orbits the Earth. The phase changes every night, and the difference over even two nights is pretty clear. It's possible, just, that over three nights the Moon can be almost full, then full, then a little past full. But usually the Moon is very different over the course of that much time. Really, the phase should have changed quite a bit during the chase. (01:00:25 - 01:22:50)
Factual error: If rockets were to attached to the basket of a balloon, the balloon would remain in the same spot but the basket would spin around it, due to the flexible ropes between the basket and balloon. (01:19:00)
Revealing mistake: In the battle against the Anubis Warriors, you can see the CGI sand explodes "wrong" sometimes. In other words, the sand sometimes overlaps the wrong areas, such as people that are in front of it.
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.
Deliberate mistake: When Rick and Imhotep battle man-to-man at the end, in one shot above them you see steam blasting from the side of the screen. Problem is that when the angle changes, it's clear that area is just empty space, so the steam came from nowhere. (The DVD commentary mentions that they always had people with flame-throwers and steam-machines right off-screen to blast fire and smoke to make certain scenes more dramatic).
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.
Revealing mistake: I can go with Alex making sandcastles to let his parents know where to go, but if you pay attention, the scale in each castle is perfect in every detail, and there are portions which would just be too hard to make by hand.