Continuity mistake: In the scene where Imhotep's priests first pop up, Jonathan is on Rick's left, and the guy in black is on his right. But in that split-second shot that shows them starting to fire, they are on the opposite sides. [In the director's commentary he does mention this but says it's because they had to cut some scenes out due to time, still a mistake but has a reason.]
Continuity mistake: In the scene in the library, when Evie is putting back the books. She leans over to put the book back, but it falls to the ground and she is balancing on the ladder. When the camera cuts back to show the whole ladder, the book is nowhere to be found on the floor.
Factual error: I counted five canopic jars, one of which has the head of a lion. In ancient Egypt there were only four canopic jars - Hapi, the baboon-headed god representing north, Imseti, the human-headed god representing south, Duamutef, the jackal-headed god representing east and Qebehsenuef, the falcon-headed god representing west. None have lion heads.
Continuity mistake: In the scene where Evie & her brother meet Brendan's character for the first time, the guards bring Brendan out with both arms shackled together in front. After he kisses Evie the guards pull him away and one of his hands is suddenly free - he quickly tucks it behind his back as he's dragged off.
Visible crew/equipment: In the scene where Brendan Fraser and the Americans are talking in the bar before the water turns to blood, there is a shot of two of the Americans and Fraser. Off to the left, you can see a hand dip in and out of frame. This is the First AD's hand. The director said he was checking his watch.
Continuity mistake: When the two teams are racing towards the city of the dead on horses and camels you can see that Evy's knees are exposed as she passes Rick, yet in the next shot they are completely covered by her dress.
Continuity mistake: When Rick finds Beni in the office, after Beni says "I am immune", Rick slams him into a pile of boxes, sending stuff flying. Nothing is visible behind Beni's head, just a sheet of paper at neck level. Then after Rick says "immune from what?" we cut to a different angle - boxes are now piled much higher behind Beni, and there's a bit of paper resting on his hat.
Visible crew/equipment: During the fire/gunfight on the boat, you can see earplugs being used by the Americans while they are firing their weapons.
Revealing mistake: When Imhotep transports Evie, Beni and himself back to Hamunaptra, Evie hears the plane and looks up. Before she states "O'Connell," you can see the letters and numbers on the side of the plane are backwards, like a mirror image - it says "B5538" backwards, which resembles "BE228" (no mirror).
Continuity mistake: In the scene when they are driving through town trying to escape the mob, the car crashes, and you can see that its lights are on. When Rick helps Evie out of the car, its lights are off. A few seconds later, you can see in the background that its lights are on again.
Revealing mistake: In the initial battle sequence, in one shot you can see the special effects canister from which the sand explodes up.
Continuity mistake: Just after the bug has crawled up into the guy's head, you come back to where Rick and the others are. You see that Rick is wearing those wrist bracers. When they hear a scream they go running. In the next shot Rick does not have the bracers.
Continuity mistake: When the mummy is initiating his new slave, he offers him jewelry and shows him a sacred jar. Where did these come from? He wasn't holding them before in that same scene.
Continuity mistake: When the fat guy who gets the beetle in his brain runs into the wall, his shirt is untucked. The camera changes angle and it is tucked in.
Continuity mistake: Following the attack of the Magi on the adventurer's camp, the short sighted American's shaving foam changes from shot to shot.