Corrected entry: When Mad-Dog is preparing his meal in the plane, he is singing. However, just before he notices Yang's people approaching, he drinks from his flask one last time and you can still hear him singing.
Corrected entry: When Brendan Fraser gets stabbed by the Emperor's dagger/sword, the outbound wound is at his belly height. When his son checks his back, the inbound wound is between the shoulder blades, although the weapon came in straight. Had the blade entered from such a high angle, the damage would have been unsurvivable by means of puncturing heart and/or lungs.
Correction: The sword did come from high in his back, down and out through his stomach. The wounds are consistent with the angle of the sword stab. The whole point of going to Shangri-la was to save Rick's life, which the magical water in the pool did. He wasn't breathing when he got there, so he very well may have died on the way up.
Corrected entry: Alex could never defeat a ninja, they're masters of all forms of combat.
Correction: Ninjas are Japanese, not Chinese, therefore Alex could not have been fighting against a Ninja.
Corrected entry: Although Qin Shi-Huangdi really did oversee the first construction of the Great Wall of China (to keep out the Mongol hordes), it was originally a series of earthen embankments - not the stone wall shown in the film. (The wall as we know it was built much later.)
Corrected entry: When Evie and Rick are asked to take the artifact to China, Evie says that they are through with "espionage." However, the O'Connells were never spies; they were archaeologists.
Correction: The man asking them makes specific reference to their efforts in WWII which happened between the 2nd and 3rd films. It's entirely possible, and given their backgrounds highly likely, that they worked as spies during this period.
Corrected entry: Neither the emperor nor any of his soldiers are actually mummies, despite being referred to as such by the main characters several times. Given their field of work and areas of expertise, it is doubtful that they would repeatedly make such a mistake. A mummy is a body that is embalmed, dried and preserved, which the emperor and his soldiers had not gone through.
Correction: Incorrect. The technical definition of a mummy is a body that's been preserved in some manner, either through intentional intervention (as with the classic Egyptian mummies) or through happenstance, as with those bodies preserved by cold or by being buried in bogs or whatever. While mystical intervention isn't part of the usual real-world definition, it's certainly true to say that the emperor and his soldiers have been preserved through time, thus qualifying them for the description.
Corrected entry: Although Alex is now a college student, Rick (his father) has hardly aged since the last film. They look more like brothers than father and son.
Correction: True, they look like brothers (and in real life Brendan Fraser is only 13 years older than Luke Ford), but the way two characters look is not a movie mistake - only poor casting if it distracts from the story. And, as you mentioned, Brendan Fraser looks like he has hardly aged in the past seven years. Some people just hold onto their youthful appearance longer.
It should also be noted that this movie only takes place 13 years after the last one, and they were made 7 years apart. That's only a 5-year difference - not a tremendous amount of time at all. And in fact, in 2014 (13 years after "The Mummy Returns"), Brendan Fraser still looked pretty much identical to how he looked in this movie. He hasn't really started to show his age until the last few years and even then, he still looks quite good for his age. Some people really do just age very well.
Corrected entry: When the assassins are coming to kill the emperor in the beginning, watch as one slices a hole in the cloth from outside. The cloth has obviously been pre-scored to easily cut, as several mini-cuts are visible.
Correction: What you are seeing is where sections of the tent have been stitched together. The reason the assassin is cutting there is because it will be easier to cut along the stitches.
Corrected entry: Soon before the final battle, as the O'Connells are preparing, you see Rick and Evie emerge from behind a rock with guns in hand and cautiously watch over what is happening. In this shot, Evie is holding two pistols, and you can see that one of the pistols (should be on the the viewer's right) has a bent barrel, revealing it to be a prop.
Correction: Already submitted.
Corrected entry: At the paramilitary camp, when the Emperor is talking with Yang, he gets angry and several pieces of his terracotta face crumble off. Yet, in the next wide-shot, there is no trace of the pieces anywhere on the ground.
Corrected entry: The good guys are buried in an avalanche, then walk to Shangri La, encounter the bad guy, then travel to the Great Wall for the ultimate showdown. Their suitcases were left under the avalanche, yet they have them at the Great Wall: guns, equipment, even a suitcase of clothes.
Correction: They have a suitcase of clothes. They also had to return to the plane where they likely had more equipment stored. They didn't walk from Shangri-la to the Wall. This is a perfect example of time compression, not a mistake.
Corrected entry: When Alex falls into the tomb and lands on the terra cotta horses, we hear a "clank" as if he landed on metal.
Correction: The horse is made of bronze, not terracota, as you can see further into the movie.
Corrected entry: The position of the Emperor when he was cursed and changed into a stone is different from when he was discovered and released from the curse.
Correction: When he cames back to life, you see the face of his statue breaking, and his raised hands under it; this means that he was put inside a larger statue, still in the position he had when he was cursed.
Corrected entry: The emperor's general had all his limbs pulled out by horse at the first of the movie, but when the general is raised he only has one arm missing.
Correction: In addition to the other correction, a deleted scene confirms this. His back is broken by the horses, and one of his arms is ripped off, but otherwise he stays in (mostly) one piece.
Correction: You never actually see his limbs torn off. It's implied, but the shot moves away from him as the horses take off. He could have one arm still attached.
Corrected entry: In the movie, the three-headed dragon has wings. Oriental dragons have no wings.
Correction: The traditional oriental dragon doesn't have three heads, either. Clearly not a traditional oriental dragon.
Corrected entry: In The Mummy Returns, Alex O'Connell has blondish hair and a British accent. In the 3rd installment, Alex has brown hair and an American accent. (00:10:05)
Corrected entry: In the beginning of the movie it shows Brendon Fraser fly fishing with the year stated as 1946 or similar. Then when shooting the gun at the "soldiers" he shouts welcome to the "21st" century, but that was the 20th.
Correction: Untrue- I just watched the film and looked for this mistake, and he stops singing specifically so he can drink from his flask, and then only resumes once he has finished drinking.