Factual error: Due to the Earth's rotation, the mirror system used to light the tombs would only provide light for about 4 minutes before needing readjustment (tested by the Mythbusters). This would be fine when there were plenty of priests around to adjust them constantly but the light level never changes even though we see Rick and Evie for several minutes at a time.
Factual error: The city at the beginning of the film where the Pharaoh lives is named as Thebes, and you can see the Sphinx and the pyramids in the background, but these are in Giza, not Luxor, as Thebes is now called. (00:00:30)
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. (00:04:20)
Factual error: When the boat is burning Rick tosses his bag of weapons over the side and it floats. This despite the fact that the obviously heavy bag bangs down onto the table in front of Evie when the two characters talk shortly before the fire - it is full of heavy guns and ammunition and the thin, porous canvas cover forms a roll around the heavy contents. There is absolutely no way it would trap enough air to balance the many kilograms of metal. The bag would sink like a stone when it hits the surface of the water.
Factual error: The map that Jonathan and Evie found in the puzzle box, made out of papyrus, would be so brittle after 3200 years that handling it would cause it to fall apart. Lighting it on fire and beating it on a carpet would leave nothing but dust.
Factual error: What is the 'Royal Air Corps'? It was the Royal Flying Corps until 1918, and has been the Royal Air Force ever since. (01:09:55)
Factual error: The sacred jars containing Anck-su-namun's organs are protected by a pressurized salt acid booby trap. The beginning of the film depicting Anck-su-namun's death is set in 1290 BC, meaning the trap must've been installed at this time, but salt acid wasn't discovered until 800 AD. (01:08:34)
Factual error: When Evelyn fights Anck-su-namun, Jonathan is struggling to read some hieroglyphics from the Book of Amun-Ra. Evelyn tells him that the word he can't remember is "Amenophis." However, "Amenophis" is not Egyptian: it is the Greek equivalent of the Egyptian name "Amenhotep" - "Amun is satisfied." In short, the name "Amenophis" could not have been written in the Book of Amun-Ra (let alone have any magical power over the undead priests) simply because it's not Egyptian.
Factual error: When Rick is running from the horsemen at the begining, he stops in front of a pillar and waits to be shot. We see the horses round the corner and rear back in fear and one falls over. We then see Rick again and then cut back to the horses, which have all disappeared. There's no way a horse could get up and run off that fast with a rider. (00:10:55)
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.