Character mistake: When LT. Col. Robert Neville's wife fails the eye scan and he's ordering the soldiers to scan her again, the soldier that escorts them to the scan point, and later to the helicopter mistakenly uses Will Smith's real last name. You can faintly here him identifying LT. Col. Robert Neville to another soldier as LT. Col. Robert Smith.
Audio problem: Aside from the beginning of the movie, the generators Robert has going put out a lot of noise that we do not hear during any of the other home scenes. This would make sense for the night shots as they would be off as to not attract anyone, but nothing is heard during any other daytime scene.
Audio problem: In the beginning of the movie when Robert is chasing the animals with his car, he points his gun out of the window and you can see his dog barking. But you can see the dog's mouth is moving without making any sound. (00:04:40)
Factual error: When Robert Neville is driving golf balls off the deck of the ship, he sees a deer on the mainland. He and his dog go in pursuit. In a wide, distant shot, we see the deer fleeing full-tilt down a city street, with Robert Neville pursuing at the same speed as the deer, only about 10 yards behind the animal. Impossible. A panicked deer can run over twice as fast as the fastest human being.
Other mistake: How did Anna drive from Maryland to New York, where Dr. Neville was, if all the bridges were blown out?
Answer: In the alternative ending Dr.Neville, Anna and Ethan are seen driving a car over a bridge that wasn't destroyed. The army would need to leave a entry/exit point, but it would have been heavily guarded until everyone died.