Corrected entry: At the end, when it turns out that the rain actually heals, the alien splashes water onto the cut on Rene's forehead and it heals. This reveals that it is not acid rain and the aliens were trying to stop the infection. Except Rene's cut was splashed by water about three times before and it didn't heal.
Corrected entry: Early in the movie, the guy driving Rene turns into a zombie near the crash site, and gets blown in half by Marion. Later on, he is holding himself together and walking. Even if he is a zombie, his spinal cord is snapped and he shouldn't have been able to walk holding himself together.
Corrected entry: At the end, when Rene is on Marion's veranda, there is a long shot of the house: there is nothing on the right side of the house. After we see Marion and a thousand other zombies, they are in a giant pen outside the house (It isn't another shot of the house as Rene is at the front).