Continuity mistake: When Rachel's still in the back seat of the van, while everyone is attacking Ray and Robbie in order to steal the vehicle, a large shard of glass hanging in the backseat window-frame disappears between shots. (00:54:00)
Continuity mistake: After losing the van the trio sit in the diner, and things on their table repeatedly change, such as when the gunshot rings out. Robbie first flips the coffee cup upside down, but in the next shot when Robbie turns the cup is upright, only to be upside down again when Rachel runs into his arms. (00:55:20)
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)
Audio problem: As Tom Cruise and the children walk towards the Athens NY ferry an older gentleman says, "We got it the worst, that's what I heard". If you look closely his lips do not match what he's saying. (00:56:30)
Factual error: When they're near the ferry, the fiery train zooms by. Passenger trains along the Hudson River run along the east bank of the Hudson, not the west bank, where the ferry was departing. (00:56:55)
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)
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)
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)
Revealing mistake: Dakota Fanning cannot actually see the computer generated birds that are flying high over the Hudson-Athens ferry. If you look closely her eyes stair straight ahead and not up in the air where they should be. (00:58:35)
Continuity mistake: The Hudson Ferry employee runs to the two soldiers and shouts, "Get that ramp up. Cast off. No more peopleĀ " In the next semi close-up of the crowd converging on the ferry, there is a large grey car on the dock at the base of the ramp. In the next shot that car is inexplicably gone. (00:59:45)
Continuity mistake: When Ray runs to catch the ferry, Sheryl loses her grip of him and when the soldier abruptly stops her the green hat falls behind her. Yet as the ramp rises, when Ray shouts, "There's room on the boat," Sheryl's hat is back on her head. (01:01:05)
Continuity mistake: In the overhead shot when Robbie (Justin Chatwin) is helping people get over the top of the ramp of the ferry the water in the back of the ferry is calm. A few seconds before the captain had gone all ahead full, created a water spray and even ripped off the moorings. The water should have been churned by the screws. (01:01:50)
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)
Continuity mistake: When the ferry casts off in the wideshot, there are only six people hanging off the outer edge of the ramp as it's being raised. When Robbie and another gentleman help these people over, in the close-up one of them wears tan pants, a long blue coat and knit hat, but this person was not amongst the six in the previous shot. (01:01:55)
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: On the ferry, Rachel wears the purple/pink camouflage hoodie over her orange knit sweater with the colorful stripe sleeves, as usual. Now this is where things become strange. In the first shot as the trio come ashore, she is only wearing her orange sweater, she is not wearing the purple/pink hoodie - note the clear lack of fur hood. In the very next shot as they climb a bit more and then rest, Rachel is wearing the purple/pink hoodie - note the hood, but now it's on under her orange sweater. In the underwater shots she is also wearing it under the orange sweater. (This odd mistake has absolutely nothing to do with the very long wool cloak that Rachel wears in the next scene). (01:02:20 - 01:04:40)
Continuity mistake: Whilst Robbie is helping the people onto the ferry in the overhead shot, Ray holds Rachel beside a red car down below as they are both avidly watching Robbie's heroism. However, in the next close-up Ray is just now approaching the car, amongst a greater crowd of people. (01:02:20)
Revealing mistake: When the Hudson Ferry captain casts off the large label that reads "Hudson Ferry" is on his left side of his jacket. However, in the exterior shot as he looks out the window at the whirlpool, the label is on his right side. This shot was flipped, and amusingly, the CGI guys actually retouched the words "Hudson Ferry" so it wouldn't read backwards. (01:02:25)
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)
Continuity mistake: In the interior shot, the Hudson Ferry captain walks toward and then stands at the doorway (seen clearly when he cast off earlier), as he stares at the growing whirlpool. However, in the next exterior shot he is standing at an open porthole window, staring at the water. (01:02:45)
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.