Other mistake: As Viggo Mortensen narrates, it's been years since every crop and animal has died. Trees have been dead so long they're rotting where they stand and falling over with a stiff wind or slight tremor. Yet, in the woods, there are always fallen leaves on the ground which ought to have long ago decomposed, rain or no rain. Obviously, they can't sweep out the whole forest to make a movie.Phixius
Plot hole: Despite supposedly being deprived of food, neither of the main characters (nor most of the minor characters) seem to display any signs of malnutrition. They both have all of their visible teeth, their stomachs are not distended and they seem to have a large amount of energy to travel by foot over lengthy distances.
Plot hole: The sheer quantity of people that the man and boy encounter is absurd. If the majority of people worldwide are deceased, then the Americas, where the film is set, would be nearly depopulated. They should have not encountered many, if any, people simply because the survivors would either be in isolated town or cities or they would be holed in remote rural areas. Since the two largely avoid the former and the size of the latter precludes easy visitation, they shouldn't have run into anyone for most, if not all, of the film.
The Man: Do you wish you would die?
Old Man: No. Its foolish to ask for luxuries in times like these.
The gang Member: You ain't gonna shoot that thing. You ain't got but two shells. Maybe one. And they'll hear the shot.
The Man: Maybe. But you won't. Because that bullet will be through your head and in your brain before you can hear it. To hear it, you'll need a frontal lobe, and things with names like "colliculus" and "temporal gyrus." And you won't have 'em anymore, because they'll just be soup.
The gang Member: You a doctor?
The Man: I'm not anything.