Factual error: The temperature on earth is increasing more and more throughout the movie. People have been dropping dead left and right, but somehow all plant life is still greener than ever. All the moisture in the ground would be gone by now, so the leaves and grass should be dried up.
Character mistake: Throughout most of the movie, many of the characters are seen wearing dark, heavy clothes such as long pants, sweatshirts, etc. Wearing warm clothing puts them at a greater risk of suffering heat stroke and dying more quickly, than if they wore light-colored, cooler materials.