Factual error: Neville shoots the propane tank in the kitchen, driving the darkseeker out into the hallway. Shooting a propane tank without an external source of ignition or a tracer would simply result in the tank's gas escaping and maybe the tank moving across the room. The Mythbusters needed an armor piercing rifle bullet to get through a tank and couldn't ignite it even with tracer rounds. Propane tanks are very thick walled. Bullet sparks are a movie myth. Bullets are either lead or lead in a copper jacket. Neither will spark when hitting metal.
Continuity mistake: When Robert Neville sees Fred at the end of the road and is lured into the darkseeker's snare trap, before Neville actually gets caught and hung upside down (approaching Fred from the truck and shooting at him), the position of the sun in the sky, and angles of light in the scene, change dramatically in only a few minutes time.