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).
Revealing mistake: The first night after O'Connell goes back to Hamunaptra with Evie, the camp is attacked. In one shot, when O'Connell is shooting at the attackers, he fires and then swings the gun to his right and fires again. As he swings the gun around the flare from the gun follows the nozzle as though it were, say, a spark on a birthday cake sparkler, definitely not like the spark on a gun.