Continuity mistake: When Ray heads home after work, he speeds past Manny's shop, and in the first shot there is a large white sign on the brick wall, to the left of the door (not the one over the door), which is seen later as well. However, in the next shot as Ray pulls into his driveway and when Mary Ann carries Rachel's backpack up the steps, Manny's is at the end of the street and that white sign is gone. (00:04:20)
Continuity mistake: When Ray empties the bag of bread, nine of its slices fall out, and he drops the plastic bag (the rest don't fall out). In the next shot there are seven slices total (not nine), and Ray tosses one of them near the box, since he needs only six. But a few shots later two slices appear near the box, together with the six that Ray uses to prepare for sandwiches, totaling eight. (00:36:25)
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: When Ray places the silver garbage can at the curb, he places it parallel to the other cans, but in the following shots they form a triangle. They also change position, such as the blue recycling can being directly in front of Tim's back car, yet when Robbie gets out the can is far enough away from the car to allow the back door to open freely. (00:05:00)
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: When Ray runs into the street he realizes that everyone is looking behind him, towards his house, so he turns to the right and looks over his right shoulder, but in the next shot facing his house, he's turning to the left and looks over his left shoulder. (00:13:30)
Continuity mistake: Ray walks into the living room when Rachel watches TV, sits down, asks about the splinter while still holding a coffee mug with his outstretched arm leaning on his leg and his feet on the floor, in the shot facing him. In the next shot from behind, his leg is now on the footrest, his arm lies on the chair arm, and he's not holding the mug. Then moments later something similar happens. (00:11:40)
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)
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)
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)
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)
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)
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.