When Sam is talking to the zombie trapped in the elevator, his right hand is bandaged. However, he does not injure his hand until he ventures outside several scenes later to try to rescue the stray cat, suggesting that the scenes were spliced into the film in the wrong order.
When Sam ventures out to try to rescue the stray cat he keeps the door open clogging it with the shotgun. As he comes back running from the zombies he closes the door without ever having to remove the shotgun.
When Sam's nose bleeds at the party, blood drips onto his box of tapes. However, when he picks the box up, the blood splashes have disappeared.
When does he saw off the barrel of the shotgun? Starts out as a long barrel shotgun and turns into a sawed off type shotgun.
When Sam wakes up after the party, there is blood smeared all over the walls in the building stairway. But later in the movie, these smears are no longer on the walls.
As Sam is laying the shotgun couple to rest in their bed, the body closest to him is visibly breathing as he lays jewellery on it.