Visible crew/equipment: After the plane has crashed Ray walks out of the house and towards the aircraft engine, as he walks behind the engine and before he comes from behind the engine, look at the right and to the rear of the engine, you will see a crew member move out of shot.
Continuity mistake: In Boston, Ray and Rachel discover dead red weed on a statue. If you look behind them, you can see a space between two buildings that is clear, only sky. In the next shot, when the crashed tripod is being shown, the spot where there was just sky now has the tripod legs. Compositors forgot to add the tripod in the previous shot. (01:41:55)
Continuity mistake: When Ray and Rachel crawl under the kitchen table during the lightning strikes, in the first shot Ray's entire body is behind the leg base, yet in the next shot his arms and upper body are in front of it. (00:16:15)
Continuity mistake: In one of the earlier scenes where they are in the dad's house, just before Rachel tells Ray that Robbie took the car, Rachel is slouching on the couch with her left leg crossed over her right. In the next shot she is sitting up with her hands on her face, and now her right leg is crossed over her left. (00:12:25)
Continuity mistake: When Ogilvy shouts, "Ray! Ray! Ray..." in his cellar, Ray runs to him and there's blood splattered all over Ogilvy's face, but in the next shot the amount and pattern of the blood differs. (01:29:30)
Continuity mistake: In the kids' bedroom, while Mary Ann speaks, Ray sits with the baseball cap in his hand. When the shot faces him he holds the brim with the cap facing out, but when the shot faces Mary Ann he holds the back of the cap with the brim hanging down. (00:07:05)
Audio problem: After Ray and Robbie's argument, as the soldiers drive by, when Rachel shouts, "Who's gonna take care of me if you go?" Robbie looks at Rachel and says something to her, but nothing is heard as the shot cuts away to Robbie picking her up. (00:50:15)
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)
Continuity mistake: When Ray tries to quiet Ogilvy, and then decides to kill him, the pattern and size of the sweat stain on Ray's shirt differs repeatedly in the following shots. (01:30:45)
Continuity mistake: In the scene right after Ray kills Ogilvy, Rachel finishes singing her song and as she's taking off her blindfold you can see that her hair is all messy and dry. In the very next shot her hair is all wet and neat. (01:33:00)
Other mistake: During the ferry scene, after Ray watches above at the aliens, there's an aerial view of the harbour and afterwards a shot of a crowd staring at the sky. Watch for a man on the right with a red hat taking a quick peek at the camera in front.
Factual error: The minivan that Ray drives is shown several times with its rear-most side window rolled all the way down. No such ability exists on any year/model of Chrysler-built minivan. They are either fixed (non-opening) or hinged at their forward edge to open like a 'flap' at the rear edge. The glass isn't broken, as it appears/reappears depending on the scene, and no such customization is possible as such a large window has no place to go down to due to the wheel well location directly below. The windows were obviously removed so the camera could move through the glass without having to use digital effects to go through the window, but in doing so, they created an obvious mistake.
Continuity mistake: When Ray speeds past Manny's there are long shadows cast, but in the next shot when he pulls into the driveway shadows are gone. When he, Mary Ann and Tim speak the shadows are back only to disappear in the next shot, and so on, all depending on camera angle. (00:04:10)
Visible crew/equipment: During the ferry disaster, while Ray, Robbie and Rachel are in the water, when the car heads toward them a vertical cable is visible at the left side of the car, beside the side view mirror. (01:03:30)
Continuity mistake: On his way home, when Ray's car turns the corner, in the first shot as he passes the parked cars across from Manny's, the last of which is a red pickup, but in the next shot it's a white pickup. (00:04:05)
Factual error: When Robbie runs up the hill to watch the army attacking the tripods, Rachel is left standing by a tree and a squadron of AH-1 gunships flies overhead to attack the aliens. However, these helicopters don't have any landing struts (certainly the one nearest the camera doesn't), but all models of the AH-1 had fixed struts.
Continuity mistake: While the aliens rummage through Ogilvy's cellar, Ray and Ogilvy have a tense, silent tug-of-war with Ogilvy's gun, and their arm and hand positions differ between consecutive shots. (01:27:15)
Continuity mistake: Mary Ann pulls the cord out of the iPod and holds it with the screen facing her palm, as she explains about Robbie's Algeria report. However, in the next shot the iPod is held in her hand the opposite way. (00:06:55)
Visible crew/equipment: When the first ship is beginning to emerge from the ground, we see a shot of two men on top of a swaying scissor lift, notice in the close-up there is nothing between them. When the shot cuts to a wide view, a camera covered by tarps appears between the two men.
Continuity mistake: In the first shot as the church sustains damage, a long massive crack rips up the asphalt and concrete sidewalk under the streetlight then makes its way up the building. However, in the next shot, even as the church receives more damage, the previous damage to the asphalt and concrete is gone and there are actually people standing around that streetlight. (00:22:15)
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.