Continuity mistake: At the start, Robert Neville drives into some yellow poles only to drive into them again in the following shot.
Revealing mistake: When the taxi goes over the edge to capture Dr. Neville, you can see that the underbody of the car as it goes over is a flat painted surface and not the underside of what should be there (i.e., muffler, gas tank, etc.).
Factual error: The grenade Robert Neville uses at the end of the movie has a blue and brown colored lever. This feature is only used on the M69 practice grenade and not the real M67. The lethal grenade lever is completely green.
Revealing mistake: When Dr. Neville first approaches "Fred" at the trap, we see the taxi from both sides as it sits on top of an intact railing. When the taxi starts to teeter over, there is now a section missing in the bridge that was cut out for the car to fall properly off and be at the right angle to "teeter" as well.
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.
Factual error: When the monsters breach Neville's house, in the scene where one of them is tearing up the ceiling while Anna and Ethan hide under the cabinet, Neville runs in, grabs what appears to be a Beretta M9/92f type handgun, and proceeds to shoot the monster several times. As he delivers the final "kill shot" the handgun is in double action (hammer is down). Unless Neville manually de-cocked the hammer, the weapon should be in single action with the hammer back, from the previous shots fired.
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.
Factual error: Robert Neville is a Lieutenant Colonel but his insignia in the shots portraying him in his military dress uniform are golden, which indicate the rank of Major.
Continuity mistake: In the scene were Robert Neville is hanging upside down from the trap that he walked into, he starts to bleed at the back of his head after hitting his head on the floor. However, the next day when Robert shows Anna his Bob Marley CD, there's a shot of the back of Robert's head after he turns the volume up, an there is no cut, graze or wound visible.
Factual error: After three years, the gasoline/fuel supply used by Will Smith would have been degraded to the point of being completely unusable in a combustion engine or generator. Depending on the storage method, gas typically beings to break down after only several weeks. Even with a fuel stabilizer additive, after three years of storage the gas would quickly ruin the fuel injectors of any engine.
Continuity mistake: The shirt Neville wears changes throughout the shots of him closing up the windows the first evening after he met Anna. They probably re-used shots from the first time he's seen doing that in the movie.
Character mistake: When LT. Col. Robert Neville's wife fails the eye scan and he's ordering the soldiers to scan her again, the soldier that escorts them to the scan point, and later to the helicopter mistakenly uses Will Smith's real last name. You can faintly here him identifying LT. Col. Robert Neville to another soldier as LT. Col. Robert Smith.
Continuity mistake: During the scene when the dogs are let loose to chase the injured Dr. Neville, the beam of sunlight is first 1-2' wide, then about 4-5' wide when the dogs try to cross, only to go back down in the next shot to about 1' wide.
Audio problem: Aside from the beginning of the movie, the generators Robert has going put out a lot of noise that we do not hear during any of the other home scenes. This would make sense for the night shots as they would be off as to not attract anyone, but nothing is heard during any other daytime scene.
Factual error: At the end of the movie when Neville is shooting at the darkseeker, he runs out of ammo and you hear repeated clicking as if the gun kept trying to shoot. He has an M4 and continuing to pull the trigger will not result in a click each time. You'd have to pull the charging handle each time you pulled the trigger to get a click.