Continuity mistake: The pharaoh realises that his mistress Anck Su Namun was touched by another man because the body paint on her left shoulder is smudged. However, the way it is messed up differs drastically between when it was being smudged by Imhotep and when the pharaoh points to it. (00:02:40)
Continuity mistake: When everyone jumps off the burning river boat it is heading away from them, but in the next shot the boat is just starting to pass everyone while they are reaching the shore.
Factual error: In the scene where the Americans find the chest containing the canopic jars, the egyptologist reads the text from left to right. However, the text actually begins from the middle and goes to both directions: one sentence goes from right to left and the other from left to right (this was very common in Ancient Egypt as hieroglyphs could be written in both directions) So, as he started from the left, the highly trained egyptologist was actually reading the inscription backwards.
Other mistake: At the hotel after the American has been relieved of his eyes and tongue and the motley crew realises the "waters" have turned to blood, Rick encounters Evelyn outside and behind her is a sign for "offices" that seems a bit too modern for the times.
Continuity mistake: When Imhotep takes Benny as his servant, he holds out some gold items in a very skeletal hand (where he had the gold when he had no pockets is another matter). When O'Connell and friends are leaving Hamanoptera, Imhotep's hand comes out of the sand. You can see that it is either a prop hand, or a crewman's hand in a glove, as it is thicker than it was a few moments before.
Continuity mistake: At the beginning of the movie when Evie is putting the books back, she is standing on the ladder leaning against the bookshelf and the top rung of the ladder is at mid thigh. When she reaches and the ladder stands up the top rung is at breast level. (00:13:00)
Continuity mistake: In the scene where Imhotep's sarcophagus first falls through the ceiling Jonathon is practicing his golf swing a few feet away from Rick and Evie. When the sarcophagus falls exactly where Jonathon has been standing, Jonathon is suddenly 20 feet away without having had a chance to move.
Continuity mistake: In the scene where the group is waiting on Evie to decipher where the black book is, Jonathan runs out to get the car and it's pointing one way. When everyone runs out to get in the car, it's now pointing on the opposite direction. There's no way he could've turned the car around that fast.