Revealing mistake: In a wide shot of the supposedly deserted city, car and bus traffic can be seen in the middle background at the left of the frame, driving along the roadway. (00:27:25)
Continuity mistake: Near the beginning, Neville sees the sun just above the horizon, about to set, and he flees for his shelter, racing through city streets. As the camera repeatedly cuts to follow him, it's obvious that the sun is still high in the sky, nowhere near sunset. In fact, the sun's position is constantly changing from one shot to the next all throughout this film.
Revealing mistake: Watch when Dutch throws the flash grenades during the stadium scene. When seen igniting on the ground, they are positioned closely behind large boxes and other bulky debris, to hide the special effects used to create the light. It would be virtually impossible to toss the grenades any distance and then have them drop down into place so perfectly on the far side of the conveniently placed obstacles.
Factual error: In the opening scenes of a city deserted for several years, lots of smog can be seen at the base of the mountains in the distance. Smog is entirely man made and would disappear completely in a matter of days once all human activity stopped.