Continuity mistake: After the ferry disaster, the trio swim to shore and climb up the bank. In the side shot Ray holds Rachel's left arm and Robbie holds her right arm. However, in the next back shot Ray holds Rachel's right arm, and Robbie holds her left. (01:05:00)
Continuity mistake: When Mary Ann is speaking in the kids' bedroom, her body is facing Robbie, and when she says, "It would be nice if he were doneĀ " in the close-up of Ray her entire body is turned the opposite way, facing the TV, but in the next shot she's turned toward Robbie again. Additionally, the amount of blue sweater visible under her grey coat changes in consecutive shots, during her visit. (00:07:00)
Continuity mistake: When Ray steals the van the front windshield is dirty and has water spots all over. Then in the following shots, it alternates between being entirely clean, or clean only in the area that wipers swipe clean. (00:30:10)
Other mistake: In the scene where the aliens appear to throw the Bayonne Bridge off its pylons and the eighteen-wheeler crashes on top of the apartments, look closely on the far right. There is a red car, and right above it is a round black object above the hedge. As the scene continues, it starts to fade away into the red house behind it.
Continuity mistake: In the end of the movie before his son runs to him Tom Cruise is standing in the sunlight. In the next shot there is no direct sunlight. (01:48:00)
Continuity mistake: When Rachel watches TV, there are things such as books and take-out food on the coffee table, which change position in following shots. (00:11:05)
Continuity mistake: When Ray runs across the street shouting, "Manny! Manny!" a guy with a hat and plaid jacket passes by, as the shot ends. At the start of the next shot, the same guy is now near Manny's garage door and air hose. (00:18:55)
Continuity mistake: After their long drive into the country, Robby and Ray begin arguing at the side of the road. In one shot as Robbie waves his arms, Robbie's blue boxers and pants are on so only the boxer's red waistband is seen. In the next shot, as Robbie still waves his arms, both the pants and boxers are pulled higher up, so now only a bit of the blue boxers are seen. (00:49:20)
Other mistake: When Dakota Fanning and Tom Cruise are in the house being searched by the alien probe, they use a mirror to shield themselves. We can see a diverging beam of light coming off of the probe, and therefore we should be able to see a related beam coming off of the mirror, widening even further. No second beam is visible.
Continuity mistake: When Ray is taking the van he's having a heated exchange with Manny, and Robbie's window is closed. However, just as Ray anxiously speeds away Robbie's window is wide open, for the dramatic close-up on Robbie's side view mirror, showing Manny's vaporizing demise, and when Robbie ducks his window is closed again. (00:31:10)
Continuity mistake: When they leave Mary Ann's house in the minivan, Ray has Rachel in his arms, and the pink cord on Rachel's sweater disappears between shots. It's hanging on her arm, under Ray's chin, the first time he tells his son to get in the van, but it's not there the second time, and it's back on the third. (00:46:00)
Continuity mistake: In the van, when Ray says to Manny "Get in, or you're gonna die," the next shot shows Rachel's face with tears in her eyes saying "What do you mean?" with a very distressed look. In next shot from the left side of the van, you can see Rachel on the right side of the screen with a different expression on her face, looking more calm, yet, in the next shot the look on her face is distressed with the look of crying again. (00:31:00)
Continuity mistake: When the tripod tosses the pickup truck, with Ray in it, it lands upside down and the windshield shatters. The size, shape, and characteristics of the hole and breakage greatly differ between shots. (01:36:25)
Continuity mistake: When Ray finally arrives at his ex in-laws' house, in the shot from behind Mary Ann, Ray passes the two cars on his right and is near the corner, but a few shots later, when Ray puts Rachel down, he is standing beside the first car, back up the street. (01:46:55)
Continuity mistake: The airplane wreckage and debris all around Mary Ann's house differs between shots. (00:42:00)
Continuity mistake: After Ray walks out of Mary Ann's destroyed house, he sees Max on the airplane wreckage. In the first shot Max's body faces out, towards Ray, as he drags the food locker, but in the next overhead shot Max's body faces the seats. His position and things on the plane also differ. (00:42:40)
Continuity mistake: When Ray places the blindfold on Rachel's eyes, while in Ogilvy's cellar, its position and folds differ in consecutive shots. (01:31:40)
Continuity mistake: Ray places the box of groceries down on the island counter in Mary Ann's kitchen. In the following shots the box (and goods inside), ceramic pitcher with utensils, tray of napkins, peanut butter, bread and bag, repeatedly, and very noticeably, change positions. (00:35:50)
Continuity mistake: As the tripod is attempting to have Ray sucked up, the soldier begins pulling Ray's arm down, trying to free him while he himself is being raised in the air. In the following shots, Ray's rescue is shown from three different angles, and each angle is distinctly different from the others. (01:40:00)
Continuity mistake: After Ray finds out that Robbie took the car, he runs into the street. In the first shot he stops at least 6-8 ft away from the solid double street lines to turn, yet in the next shot he's standing right on the double lines. (00:13:25)
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.