Other mistake: Bruce Willis's character has a very clear scar on his forehead in some scenes (I think it's on the right as you look at him) - doesn't really fit the whole "unbreakable" theory. You'd think they could have put some makeup on it...
Other mistake: David is supposed to be immune to disease, which is why no one can ever remember him getting sick, but at a number of points in the movie the Chicken Pox scars on Bruce Willis's face are visible. If he was immune to disease, he should never have had Chicken Pox. I noticed this best in the scene where he asks his wife if she can remember him getting sick, look at his right temple.
Suggested correction: The scars could easily have come from something else. His skin is not impervious to damage, so extreme trauma would break his skin and leave a scar.
David's skin is impervious to damage. After waking up in the hospital after the train crash, Dr. Duban tells David that not only does he not have any broken bones, but also that he doesn't have a single scratch on him. If his skin wasn't impervious, then he should have suffered major cuts and lacerations from the wreck. Even during the flashback of the accident when he and Audrey were teenagers, Audrey is show bruised and bloody while David is completely unharmed.
Suggested correction: David is not "immune" to disease. He has a much higher resistance to disease.
Suggested correction: According to a previous explanation, David's "unbreakableness" is limited to his bone structure and general health. His skin can break and heal naturally. See the correction on why the scar from his ear piercing shows.
Twotall
When David wakes up in the hospital, Dr. Dugan tells David that he doesn't have a scratch on him. If his unbreakableness was limited only to his bone structure, he should have woken up covered in scratches from not only the force of the collision but also from the glass as he was sitting next to the window.