Audio problem: After Ogilvy is dead, when the alien snakelike giant eye surprises Rachel she begins screaming. As Ray hacks away at it with an axe, she stands at the wall screaming, and when the last long scream is heard, her mouth is closed just before Rachel runs to the stairs. (01:34:30)
Continuity mistake: In Boston, when Mary Ann lifts Rachel, she holds her daughter then turns to face Ray and says, "Thank you." Behind them, up the stairs, the left front door is already wide open, yet in the next shot both doors are closed until her parents dramatically open them. (01:47:30)
Continuity mistake: When Ray is at the News Producer's van, she grips and starts to push the videotape with her palm up, but in the next close-up it's the opposite, she's pushing it with her palm down. (00:44:00)
Continuity mistake: Ray and Rachel are about to walk down into the basement at Harlan's house, the door on the right of the bulkhead is open and the door on the left is closed. In the view from the basement you will notice that the door on the left is open and the door on the right is closed.
Continuity mistake: Whilst Ray is trapped in the upside down pickup, he looks through the windshield hole directly at Rachel, who stands on higher ground. In the next shot from behind Rachel, the landscape, wood fence, etc., to her sides are entirely different from the previous shot. (01:36:35)
Continuity mistake: When Ray and others are crouched beside the hole in the asphalt, the guys around him and the way he holds the freezing piece of asphalt, differ between consecutive shots. (00:20:50)
Continuity mistake: When Ray reaches the red church at the beginning of the film, the sun changes positions three or four times behind the tower of the church. (00:20:10)
Factual error: The New York Central Railroad's West Shore Line train tracks depicted in the scene with the burning train in Athens NY is not accurate. Tom Cruise and the children walk right onto the ferry terminal after crossing the train tracks, but in reality the tracks are over two miles to the west of the Hudson River shoreline. (00:57:20)
Continuity mistake: As Mary Ann's house shakes during the alien attack, when Ray shouts, "Where do we go? Robbie, this is your house. Where do we go?" Robbie is hugging Rachel. In this shot Rachel's hair is extremely curly, whereas in previous and following shots it's barely even wavy. (Most likely she's the body double.) (00:40:15)
Continuity mistake: In the countryside, after Rachel witnessed many dead bodies float down the river, and they head back to the van, an Army Convoy rolls by. While rolling by, Robbie starts jumping up and down waving his arms. His pants creep down enough to see his butt crack and reveal his red underwear. The next moment, picking up at a different angle, his backside is now covered by his grey underwear. (00:49:25)
Factual error: As Tom Cruise and the children walk on route 385 towards Athens NY they pass a sign saying that the town is 1 1/2 miles away just before they cross a bridge. In reality there's no bridge to cross 1 1/2 miles from Athens NY on route 385. (00:56:20)
Factual error: The burning train scene as Tom Cruise leads the children towards the Athens NY ferry is fantastic. The only problem is route 385 does lead them to the center of town but it doesn't intersect a rail line. (00:57:00)
Continuity mistake: As the boy runs up the stairs to the second level of the ferry Tom Cruise yells, "Robbie". If look closely there's a large speaker in front of him that can be seen in the first two shots but then disappears for the rest. (01:01:50)
Factual error: The burning train scene in Athens NY is a classic, but not accurate. New York Central Railroad's West Shore Line tracks have been abandoned for years and are not usable. (00:57:15)
Continuity mistake: When Rachel says Ray should get TiVo, she is looking at her hand. When it cuts she is suddenly resting her head on her hand. (00:11:55)
Continuity mistake: When Ray runs behind his house to have a better view of the sky we can see from behind that his hood is blowing in the wind. When it cuts to a shot in front of him his hood is still. (00:13:40)
Continuity mistake: Ray leaves Ogilvy's house to search for Rachel and soon runs into the pickup truck, just before the tripod's tentacle lifts it. The massive red roots growing all over the pickup are very different in the next shot, as it's lifted in the air. (01:36:20)
Continuity mistake: Right after the Hudson-Athens ferry boat captain guns the engines you can see there are no ropes between the boat and the pier. In the next shot there are as the pilings are ripped off into the water. (01:01:55)
Continuity mistake: When we see the Hudson-Athens ferry boat captain from outside of his bridge, the video screens in each window above his head are positioned well below the tops of each window. In the next shot they're higher and we barely see them. (01:02:35)
Factual error: When Ray comes out from the basement to survey the damage, he sees a crashed plane. As he walks past the fuselage, there is a square window on the cockpit. Planes don't have square windows because to corners were proven stress points from the explosions of two De Havilland Comets in the early 1950's. After the investigations, all plane windows were deemed to be rounded to prevent another such incident.
Chosen answer: In the original George Pal version they were Martians and the reasoning for what they were doing was never explained. In this version, it's never explained where they come from, but their mission is simple, to eradicate human life from Earth, and use our bodies to fertilise the planet, probably so that they can colonise the planet for themselves.
GalahadFairlight
If it was to eradicate us they could have done that millions of years back, why now, so that doesn't add up.
You want to grow the substance (people) that grows your food source before using it. If they waited too much longer, they'd have a harder time because we'd have the technology to fight them back.
The reason which was apparently provided by Wells was that Mars was dying by lack of natural resources and that Martians needed a new home and food source.
They were waiting until the population grew large enough to sustain terraforming efforts. As they used our bodily fluids seemingly as a primary material for their terraforming.
It's an assumption that they could have eradicated us millions of years ago (which by the way would be long before we even existed). Maybe they didn't have the ability to transport themselves, only the machines. Maybe the original aliens all died. Lots of other options why they couldn't have done it.
They probably needed to wait for us to produce enough humans to use as fertilizer. Doesn't make sense to try to use several million bodies as fertilizer back then vs now with billions of people.