When Neville fires his gun (a Colt Law Enforcement Model 6920 Carbine) at the dark seeker in the house and runs out of ammo, the weapon can be heard clicking several times. An AR-15 type weapon will not click when the magazine runs empty. The bolt carrier group will lock to the rear holding the hammer down; pulling the trigger has no further effect.
When Zoe Neville fails the virus screening at the Brooklyn Bridge checkpoint, Lt. Col. Robert Neville gives a direct order to the soldier administering the test, demanding that he repeat the test. Robert may have outranked the soldier, but failing the requirements of the chain of command, the soldier would not be required to obey the order.
Robert Neville tells Anna that there were 6 billion people on Earth, 90 percent were killed by KV outright (5.4 billion) and that there was one percent immunity. From that he arrived at twelve million immune and 588 million dark seekers. However, if calculated from the 600 million remaining (the ten percent that did not die outright) one percent would be six million, not twelve million, leaving 594 million dark seekers, not 588 million. The numbers he arrives at would be correct with 2 percent immunity.
If you look out of the car window on the driver's side (at roughly the 3:18 mark) just after Neville grabs his gun, you will see a silver and red car parked (silver on left, red on right). Neville continues driving down the road and away from the parked cars, but at around the 4:00 mark, when you get another clear shot out his window (over his shoulder), you can see the same two cars parked, yet he is not in the same area where they first appeared.
After his first swing, we can clearly see only 3 balls remain on the SR-71, and they're immediately beside the dog. But when the shot reverses, he hits another without resetting a ball, or taking a step forward, or disturbing the dog.
When Robert Neville has his flash backs with his family in their SUV a zombie smashes his face against the window creating a blood splatter mark on the passengers window. But in a later flashback the blood splatter is gone.
When Robert is hitting golf balls from the aircraft carrier and uses his carbine to scope the running deer, the flashlight on his weapon is on (his) left side of the barrel. In the subsequent scene when he enters the dark area, the flashlight moves to the opposite side.
When Neville first sees Fred on the bridge, the taxi used for the trap is hanging over the bridge railing close to the front wheels. But when the trap is sprung, we see the taxi from another angle, and a section of the railing is gone and the taxi is much further out over the edge.
In the first scene in Neville's apartment, he is seen watching TV. The TV has 3 A/V connectors plugged in the front, presumably from an adjacent device. In later scenes, although the same devices are still there, the A/V connectors are now missing.
After Robert Neville and Sam escape the warehouse with several of the infected humans now thrashing wildly on the street in reaction to the sunlight, Neville gathers up and restrains Sam. But after releasing Sam, and as he continues to observe the the dying infected, Sam can be seen in the background playfully bounding out of the scene.
When Neville is in the dark warehouse looking for Sam, he enters a bathroom. When his flashlight hits a mirror, he ducks and shields the light, but there is still a circle of light from the camera behind him.
When Robert Neville is about to enter the dark area to chase after Sam, and as he is swinging his gun from side to side, for a split second, a camera's shadow can be seen sweeping towards the left as he is doing so.
The central spans of the Brooklyn and Manhattan Bridge are destroyed, but the towers and side spans remain standing which is impossible. If you cut the suspension cables, the whole bridge will collapse. The towers are not strong enough to hold the cables - the weight of the side spans will pull them down.
When Neville opens the drawer in his lab and grabs the grenade, it is shown as a fragmentary grenade. However, when he sets it off, it explodes in a burst of flame. Fragmentary grenades explode without large flames and throw shrapnel. The explosion shown would have been from an incendiary grenade, which looks like an aerosol canister and not the rounded frag grenade.
The plague allegedly strikes in 2009. On one of the news tapes Neville is watching, the bottom news ticker indicates that the "Patriots defeated the Giants" for the second time that season. The Patriots would only play the Giants once every four years, and never twice in the same season, unless the two met in the Super Bowl, and the ticker doesn't indicate that it was the Super Bowl. The Patriots and Giants, in fact, played each other twice in the 2007 season (one in the regular season ans once in the Super Bowl), therefore they won't play each other again until 2011, unless they meet in the Super Bowl.
The sight on Neville's carbine is a US ACOG sight, yet, when the camera shows the view through the sight, the reticle is from a Soviet PSO-1 sight and some other eastern block sights. There is no ACOG sight with that reticle.
Neville is able to drive around the city after 3 years of isolation. Gas starts degrading in three months in the tank of the car, but in a closed tank (such as those at a gasstation) it can be kept for several years. The degradation is due to the fact that it evaporates and mixes with oxygen, but that doesn't happen in an airtight tank.
At the beginning, the bridges are blown up to prevent access to Manhattan. At the end, they are seen driving off Manhattan on a bridge. However, only two bridges are shown being destroyed. There are many other bridges that may have been left intact in order to allow (yet minimize) access. (Only the Brooklyn Bridge and Manhattan Bridge are shown to be destroyed. Other bridges, such as the Williamsburg and Triboro, might have been kept but blocked. The intact bridge at the end of the alternate version appears to be the George Washington.)
When Anna rescues Robert using her vehicle, she said she drove all the way there from Baltimore. But since all the bridges were already destroyed, it's not clear how she got her vehicle onto Manhattan Island. The only bridges that are definitively shown to be demolished are the Brooklyn and Manhattan bridges. The George Washington Bridge could be intact (and is shown to be at the end of the alternate version). However, she could also have taken a small boat to the island and resupplied there. The island is full of cars and goods, and there's no indication that the car she was driving on Manhattan is the one she drove from Baltimore. In fact, the car she's driving when she reaches Vermont at the end clearly is new, suggesting she had to get another car after leaving the island.
When Neville wakes up in his apartment after Anna rescued him he enters the kitchen with a gun in his hand. In one shot Anna has a white top and a pink bathrobe. Seconds later she wears a green shirt. However, this was Neville imagining Anna and Ethan as his wife and daughter before snapping back to reality.
When Neville is eating cereal he pours on milk. But three years have passed so there would be no fresh milk. However, Anna later mentions that the eggs are powdered eggs, so it is certainly powdered milk that Neville puts on his cereal.
During the beginning scenes, where the camera pans over the city, during one pass over a road that runs horizontally across the picture, if you look closely people can be seen walking close to the buildings.
When Robert Neville goes to sleep the first night, he is adamant about shutting all reinforcements on the windows. Yet when he wakes up in his bed in the morning, we see the sun shining in through the unbarred windows. As proved later in the film, he cannot remove the barriers from the windows until dawn has broken properly.
After Neville hits the first golf ball, the camera pans around the aircraft carrier. At the end of the street on the left of the screen, between the on-ramp and the white building, there is a quick shot of a vehicle moving right to left.
Neville records that his infected female subject has a "PaO2 300% of normal" - not possible as she was clearly breathing air, in which the partial pressure of oxygen is only about 157% of that in normal arterial blood.
A little after the scene where Robert was ensnared and then he cut himself loose; The streak of sunshine on the street preventing the monsters reaching Robert becomes narrower as the sun sets. For the sunshine on the street to become narrower as depicted in that scene, the sun would have to have been setting behind Robert or the alpha male. The was sun setting into the distance on Robert's right side and the alpha male's left.
When Robert finally talks to the mannequin after Sam's death, he approaches "her" from behind. In front of the mannequin there are many movies on the shelves, but when Robert starts talking to "her" being by her side, the same movie cases appear next to Robert.