Deliberate mistake: The shot of Ardeth and his companions watching Rick leave Hamunuptra at the start of the film is reused again when Rick, Evie, Jonathan and the Warden re-enter Hamunuptra, although the reused shot has a blue filter over it. Ardeth even says exactly the same line he says in the original shot, but it is subtitled differently.
Deliberate mistake: When they first enter the tomb, Evie moves a mirror which deflects light onto a load of other mirrors. You can see the beam bounce from mirror to mirror as it goes along the chain. Whilst this looks cool, the speed of light is far too quick for this to actually be noticeable as shown.
Chosen answer: The tattoos on his forehead are the Egyptian Hieroglyphs that spell "Underworld", and the ones on his cheeks are the Egyptian Hieroglyphs for the word "truth." All Medjai males get these tattoos as part of the coming-of-age rite, when they turn sixteen, of which the most important is the tattoo on their right wrist (which Rick O'Connell also has) that marks them as "warriors for God." Other tattoos specific to Medjai males are on their arms, forearms, hands, pectorals, shoulder blades and beneath the navel - the tattoos on the nose and chin are no longer used, since the time of Seti I. Medjai females only get the wrist tattoo when they come of age, but are not marked with any of the other symbols that are particular to men. Fun fact: If the Medjai - male and female alike - shows any sign of pain or cries during the tattooing process, it is considered that they have brought shame to their family.