Continuity mistake: When Ray and the kids are driving down the freeway, there are several cars stopped randomly so they have to swerve a lot. But when it shows them inside the car, talking, they are driving perfectly straight. When it shows them from the outside again, the random cars are back and they start swerving again.
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: When Ray and the two guys head to the spot with 26 strikes, there is a crowd of people walking behind them in all the close-ups, yet, in the overhead shot hardly anyone is behind them as they approach the hole, and there was no time for the crowd to move ahead or disperse. Additionally, some of those people who are already positioned at the center of the crowd, around the hole, were just behind Ray in the previous shots. (00:19:35)
Continuity mistake: When Mary Ann and Ray walk into the kids bedroom, Robbie is lying on his bed with books, etc. In the next shot his position, pillow, etc., differ, and there are different books on his bed. (00:07:00)
Continuity mistake: When Ray and Rachel are in the backyard, just before the lightening starts, she says, "I don't wanna go by myself," and there are no leaves on the barbeque. In the next close-up, as they stand near it, there are leaves on it. (Not due to wind.) (00:15:35)
Continuity mistake: When Ray is in his backyard and says, "That is so weird," he's standing well away from the fence that is near the female neighbor holding the baby, but in the next shot he's right beside the fence talking to her. (00:14:35)
Continuity mistake: After Rachel tells Ray that Robbie took the car, when Ray runs down the steps long shadows are cast. When Ray turns around to see what everyone is looking at, all shadows are gone, only to reappear in the next shot. Then when Ray runs to his yard, the angle of his shadow is entirely different. (00:13:30)
Continuity mistake: Just before the news van leaves, in the shot facing the News Producer she asks Ray, "Hey were you on that plane?" then extends her left arm to touch Ray with her left hand. However, in the shot facing Ray, she has her right arm extended to touch Ray. (00:45:15)
Continuity mistake: When Ray breaks the window with the baseball, the characteristics of the hole differ between exterior and interior shots. (00:10:10)
Continuity mistake: While up in the tripod's "holding cell" for humans, Ray spits the circular grenade pins into his hand, and their positions on his palm differ between the two consecutive close-ups. (01:40:15)
Continuity mistake: When Ray runs out of the house and meets up with Robbie, the cars and pedestrians very noticeably differ between consecutive shots while they talk. (Not due to cars being pushed.) (00:18:15)
Continuity mistake: Once Ray takes the piece of pita with hummus and eats some of it, the rest of the pita and the way he holds it differs in following shots. (00:12:30)
Continuity mistake: As the News Producer speaks with Ray amidst the plane wreckage, her black bag, her actions, and how she hold things such as food trays, change abruptly in consecutive shots, while Max searches for food in the locker. (00:43:15)
Continuity mistake: As the hole begins to crack in the overhead wideshot the crowd moves back, and the florist is at the top center of the screen with hardly anyone near it. Yet, in the next close-up, suddenly there are dozens of people standing in its path, as the crack dramatically approaches the florist. (00:21:15)
Continuity mistake: When the asphalt quakes in front of the church in the overhead shot, just as the huge gush of water shoots up from the ground, in the foreground a person is leaning way out their window aghast at the goings on. In the next shot that person is gone and the window is shut. (00:22:45)
Continuity mistake: As Ray says, "Next time you take my car with no driver's licenseĀ " he walks a good distance away from Robbie. Yet in the next close-up of Robbie, Ray's arm is still pointing up near Robbie's face. (00:18:25)
Other mistake: Mary Ann and Tim's basement, and Ogilvy's cellar are the same set, just dressed differently; note the positioning of the windows, the stairs, even the little room off to the side. Also, some items in both the basement and cellar are the same, for instance the chair. While they are in the basement and the plane is crashing, before they run into the side room, Ray grabs his gun off of that chair and later in the movie when they are in Ogilvy's basement, Rachel is in that chair when the aliens are in the cellar.
Continuity mistake: When Ray heads upstairs from Mary Ann's wrecked basement, there is a mess beside the stairs. However, in the next shot looking down, there's no mess near the stairs, furniture differs, and now there's a wicker hamper that appears out of nowhere. (00:41:35)
Audio problem: When Ray carries Rachel out of Mary Ann's destroyed house, Ray and Robbie walk past some wreckage and are near the van, yet in the next shot they just start walking past that wreckage again. Additionally, the audio is not in sync with Ray's lips in the first shot. (00:45:40)
Continuity mistake: As the asphalt begins to burst open, the Portuguese restaurant's windows shatter, and when the camera pans to the church there are very few people on the street in front of it. In the next shot, however, there is a huge crowd in front of it. (00:22:10)
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.