Continuity mistake: At the start, Robert Neville drives into some yellow poles, only to drive into them again in the following shot.
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.
Continuity mistake: At the end of the movie, when Neville is behind the glass and he is thinking and all the sound goes away, you can see the bite mark that the zombie gave him. While the shot is changing between him and the zombie, you can see the bite mark change sides of his shirt.
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 reused shots from the first time he's seen doing that in the movie.
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.
Continuity mistake: Just before Neville goes into the bank with the creatures and instructs Sam, "So look, if I'm not back by dark, just go.". The camera show Neville reaching for the dog, it changes perspective and you can see Neville reaching again. (00:31:30)
Continuity mistake: In the beginning, when Robert is sealing off his house, the height of the sun changes between shots. When his alarm sounds, it is almost horizontal but at one of the other windows it is bright sunshine, and the sun is coming from high up.
Continuity mistake: At the beginning of the movie when he is driving his Mustang, the movie is shot inside the car several times. If you look carefully you can see that the same background scene has been used twice. In the scene, you see a red wall from a building and a grey car standing along that red wall. This scene is used twice.
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. (00:46:40 - 01:14:20)
Answer: In the alternative ending Dr.Neville, Anna and Ethan are seen driving a car over a bridge that wasn't destroyed. The army would need to leave a entry/exit point, but it would have been heavily guarded until everyone died.