The codex-style books (pages, covers and binding) were not invented until the Roman Empire. Ancient Egyptians used scrolls. Therefore the Book of the Dead or book of Amun-Ra should not look like modern 'books' at all.
In the climax of the film, Evey is set to be sacrificed. When O'Connell arrives to rescue her and fights off the mummies surrounding her, the impact of him hitting them with a sword is a sword-on-sword. It should be a sword-on-flesh sound.
When the scarabs are poured over Imhotep, they appear to begin devouring him gradually, a fact which Evelyn later confirms, stating that the beetles devoured a corpse "very slowly." However, whenever the scarabs devour anyone else later on in the film, they do so almost instantaneously.
While Ardath, Rick and Jonathan are trying to dig through a cave-in, Jonathan's accent suddenly switches from English to Scottish when he says "Come on, put your backs into it!", then it returns to English for the remainder of the movie.
Evelyn says in the movie that they take out all your organs during mummification including your heart. It is believed that the Egyptians left the heart in the body because it was the seat of intelligence and feelings.
When Beni is looking for the black book in the office, he gets caught by O'Connell. When they are standing under the fan, you hear the scream of the Egyptologist. They all look toward the camera but when Beni runs out the window behind them, the Egyptologist is there.
When Rick, Evelyn, Jonathon and the Americans come across Ardath Bey and the curator in the museum, Rick, Jonathon and the Americans draw their guns. When the camera switches to the curator greeting them, Jonathon's hands come into screen as he draws his gun a second time.
When Evie is showing the curator the map that Jonathon found, she points to the bottom right of the map and says "Hamunaptra." Yet when the curator "burns off the bit with the lost city," it is the left side of the map that catches on fire from the lamp.
When Jonathan goes to start the yellow car, the car is facing away from the direction that they need to go. Within a split second as everyone else is running to get in the car, the car has turned around in the opposite direction. There wasn't enough time to turn the car around between the time that he started the car until the time that everyone else got in.
When Evie is on the ladder, she's holding a book that she tries to put on the adjacent bookcase. When she drops the book, there is no sound of the book landing, and when she leans over and the ladder is supporting itself, the book is nowhere to be seen.
When Rick, Evie, Jonathan and the Warden are beginning their first exploration, Rick pulls a gun out of his left holster when Evie informs him of where they stand is the preparation room for "the after life". When they are in the hallway and hear the bugs around them, Rick is holding a gun in his left hand and the gun he had drawn before was back in the holster.
When the Americans and the British are racing to the City of the Dead, the sun is behind the city. Their shadow goes off to the left, meaning that the sun is to the right of them. If they were racing towards the city their shadow should be behind them.
When Evie steals the Book of the Dead from the Egyptologist, his left arm is around the book and his right arm is around a canopic jar. When he awakes, after Evie has read from the book, the jar is still cradled in his right arm as he sits up, but in the very next shot it is in his left arm with no time at all for him to have moved it there.
When the Egyptologist is running through the streets of Cairo, he is wearing the same outfit that he wore before in the film: a light colored suit, black vest, black patterned tie, gray scarf, and a fez. In the beginning of the film, his fez had a tassel, but when he runs, the tassel is gone. When the mummy kills the Egyptologist and the crowd is around the body, however, the Egyptologist's body has no scarf, tie, vest, or shoes; they are all surrounding the Egyptologist's corpse. The fez is still on his head and has a tassel on it.
When Imhotep kills the last of the Americans, Beni has a considerable amount of stubble on his face and a thicker mustache. When Imhotep corners the rest of the group after they crash the car, Beni has much less stubble and a more defined mustache.
When Evey is talking to Rick by the campfire, since she's drunk, she tries to kiss him. The angle at which she's leaning changes multiple times. First, she leans to her right, then a different angle shows her leaning the opposite way. These shots alternate until the moment she passes out.
Near the end of the film, in the final fight scene between Imhotep and Rick, Imhotep grabs Rick and opens his huge mouth, but on the shot after Evvie starts reading the spell to make Imhotep mortal, his mouth is open regularly.
In one of the Hamunaptra scenes, when Dr. Allen Chamberlain shouts at the workers who then die by pressurized acid, Chamberlain wears glasses in the first 'yelling' shot, then in the close-ups, he doesn't.
When Imhotep is trying to resurrect Anak-sun-amun in the beginning of the film she is shown gasping for air as her spirit re-enters her body. Then in a wider shot she has collapsed(as if it is not in her body) even though you can still hear her. In the next close up she is gasping for air and screaming again, before her spirit leaves her for good and she collapses.
As the two parties begin their race to Hamunaptra, Rick and Beni are in the lead. Beni begins hitting Rick with his whip and is pulled from his camel and thrown to the desert floor by Rick. As Beni stands up, we see the others on their horses coming up behind. The scene then changes to a wide shot and the standing Beni is nowhere to be seen.
When Jonathon is attacked by the scarab, rick rips his shirt sleeve open to cut it out of him. In all subsequent shots, Jonathon's shirt is intact, and the bandage he is wearing on his arm bears no relation to the wounds received in the incident.
When Rick, Jonathan, Winston and the Americans are sitting at the bar in Fort Brydon (after the introduction of Winston), you can see 1st Assistant Director Cliff Lanning with his script in hand on the very left side of the screen.
In the opening scene where Rick & Beni are crouching behind the wall with their buddies ready to shoot the approaching armed horsemen and the camera is panning along the side of the galloping horsemen. In the background is a quite visible camera stand and two camera operators shooting the scene from another angle.
The ancient Egyptians believed that there was a strong link between death and the compass direction west (where the sun "died" every evening). The fact that the adventurers see the sun rising over Hamunaptra as they approach it suggests that they have been traveling eastwards from the Nile, but if the ancient Egyptians had built a "City of the Dead" they would never have built it on the eastern side of the Nile.
In the opening shot of Thebes, pyramids are visible, but there were no pyramids built around Thebes. You can also see a small version of a Sphinx in front of the pyramids, making the whole set look like Giza, in Cairo.
There are five canopic jars pictured in the chest. However, only four canopic jars were ever used during mummification. The heart was left in the body, and there for would not have had a corresponding canopic jar.
Both times, when the Arab horsemen are attacking Hamunaptra, the sound of ululation is heard. In the Middle East, this sound is traditionally made by women, and during times of celebration or grieving. It wouldn't be made by attacking men on horseback.
When Dr. Allen Chamberlain is reading the mummy's curse in hieroglyphs, he is in fact reading them the wrong way. The way the hieroglyphs were facing, they were meant to be read right-to-left. Ancient Egyptian hieroglyphs can be read right-to-left, left-to-right, or up-to-down.
When Evelyn and Rick climb down into the vault under the statue, Evelyn uses mirrors to illuminate the chamber. You see the light passing from one mirror to the next. The speed of light is way too fast to be tracked like that.
At the beginning of the movie when the horsemen are about to shoot O'Connell, all of the horsemen are holding bolt-action rifles with one hand as they get ready to shoot and you hear all of the rifles being cocked even though it takes two hands to cock a bolt-action rifle.
Imhotep fears cats because "cats are the guardians of the Underworld". In Egyptian mythology, cats were associated with the goddesses Bastet (fertility, motherhood and protection) and Sekhmet (healing) and not the Underworld.
Pharaoh's guards lock Imhotep in the sarcophagus by turning the knob to the left. Later, when Jonathan, Evelyn and Rick are opening the sarcophagus, they also turn the knob to the left. Apparently that's how the locking mechanism works.
Notice the gunshot-wound that one the Americans of O'Connell's entourage has in the car-chase sequence when he is unsuccessfully fighting off Imhotep's followers in the village. He got shot in a scene which was filmed but not shown. (Source: director's commentary on DVD-release)
The heroes use the star "key" to open three locks: Jonathan opens the sarcophagus by turning the key counterclockwise, and Evelyn turns it clockwise to open the Book of the Dead and the Book of Amon Ra. Although this inconsistency may seem strange, apparently that's how the locks work. It may also seem strange that the characters always turn the key the right way on the first try, even though they're not familiar with the locks. However, a closer look reveals that they do try to turn the key the wrong way, and after realizing that it doesn't work, change the direction. This is especially evident in the second case. Besides, there is always a 50 percent chance of guessing the right way on the first try.
When Imhotep realizes Beni is speaking "the language of the slaves", he speaks one or two sentences in Hebrew, then switches back to ancient Egyptian, but Beni still understands him. Apparently, Beni does know ancient Egyptian (given his occupation it shouldn't come as a surprise), even though not so well, which becomes apparent when Evelyn starts correcting him. So, there is no reason for Imhotep to continue to speak Hebrew.
The ancient Egyptians believed that there was a strong link between death and the west (where the sun "died" every evening). If they had built a "City of the Dead" it would not have been on the eastern side of the Nile. However, the movie never claims otherwise. The fact that the adventurers see the sun rising over Hamunaptra does mean that they approached it from the west, but it doesn't mean that they have been traveling eastward from the Nile. Apparently they circled around to the western side of Hamunaptra, so that the rising sun could show them the way. This would also explain why both parties approached the lookout point from the opposite directions (they circled the city differently).
Evey says she lost all her clothes when they were fleeing the riverboat, implying she left her suitcase on board and it burned in the fire and she has to buy new clothes in a market. However after they escape from Hamunaptra, Evey is wearing her librarian outfit from the start of the film.
When Evie and Rick are running from the flaming balls falling from the sky in the courtyard of the fort, the light is too bright for an overcast day (which we can see that it is from the shots of the sky).
When Imhotep has "assimilated the organs and fluid" of the blonde, American cowboy Mr. Henderson, and the bug crawls up Imhotep's face, you can see the edges of the appliance used to create the CG holes in his skin.
Soon after the main characters pick up their camels, there is a left to right panning shot of them riding in the desert. At least five dust marks on the camera lens are revealed as pale shadows against the sand.
When the Americans fighting the Medjai on the boat are shown from the side, one can clearly see that one of the 'cowboys' cocks his revolver and pulls the trigger several times but the gun does not fire.
When the bookcases collapse near the beginning of the film, you can see that the bookcases farthest from the camera are hollow boxes covered with fabric upon which books have been painted. The fabric ripples with each impact.
Near the very end of the movie (just after Imhotep becomes mortal again), O'Connell runs Imhotep through with a very large sword. Imhotep's hands are placed over the wound, then taken away. As he looks at his hands, he sees blood, and the blood has run through his fingers to the backs of his hands. When he places his hands over the wound again, there is no blood visible on the backs of his hands or on his body.
After O'Connell is saved from being hanged, they all prepare to board the boat and Evelyn is shocked to see how cleaned up he is. However, despite being quite brutally hanged by a rope only hours before, there are no visible bruises on O'Connell's neck.
Imhotep takes the Burns' (the American man with eyeglasses) eyes when he begins to become human again. While Burns is very near-sighted, it's been established that Imhotep doesn't receive the actual organ he steals (if he did, he'd appear to be even more of a patchwork creation). In the novel of the film and script, however, it's been stated that Burns, having poor eyesight, gave his faulty vision to the mummy when his eyes were taken. The concept was furthered in the script when Imhotep uses a pair of spare eyeglasses to fully see Burns and kill him.
When Beni is bringing bags of treasure out of the city of the Dead, he loads the bags on camels, yet only five people among the expeditions (O'Connell, Evelyn, Jonathan, the Warden, and Beni) rode camels. The rest rode horses, with the exception of the Egyptologist, who rode a donkey. The novelization tries to say that these are the French Legion's abandoned camels, but how could those beasts be still alive after 3 years?