Minor plot hole: Hanging upside-down for too long can seriously hurt you. Robert Neville however spends almost half a day hanging in an upside-down position without dying from all the blood flooding in his brain. I mean, David Blane did it for a few minutes before abandonning his act all-together in front of everyone because it was that painfull.
Major Plot hole: It is revealed that the zombies are actually quite intelligent, enough to set up the same trap Will Smith's character used to trap one of them before. However, when he is caught upside-down by a wire, the zombie leader send 3 dogs after him.......but does not join them. Again, if zombies are so bright and learn from Robert Neville's actions against them (like the trap), then why can't they use guns ? Or cover themselves in big-cloaks to travel during the day ? The movie shows them as either intelligent or dumb depending on what the script needs.
Plot contrivance: The 3 zombie dogs that are sent to kill Robert Neville have to wait until the thin line of light is gone before running to attack him. Even though they could have run right through it with superfluous damage.