Continuity mistake: When Ray throws the bread with peanut butter at the window it lands dead center. However, in the next close-up it's off to the side of the glass, in order for Ray's and Rachel's reflections to be seen clearly. The slice of bread differs as well. (00:37:15)
Continuity mistake: In Mary Ann's basement, when Ray and Robbie are looking out the small window, as the aliens begin their attack, Rachel runs over and Robbie hugs her tightly. However, in the next shot of Robbie and Ray, Rachel is gone, yet in the very next close-up Robbie is still hugging Rachel. (The shot of Ray and Robbie without Rachel is cut from a previous shot, before she runs to them.) (00:39:30)
Continuity mistake: As Mary Ann's house shakes during the alien attack, when Ray shouts, "Where do we go? Robbie, this is your house. Where do we go?" Robbie is hugging Rachel. In this shot Rachel's hair is extremely curly, whereas in previous and following shots it's barely even wavy. (Most likely she's the body double.) (00:40:15)
Continuity mistake: When Ray and the kids are hiding out in the mom's basement, they take cover in a small utility room that goes pitch black when Ray shuts the door - even though there are windows above the girl's head that emit light before he shuts the door and after he opens it to leave. (00:40:35)
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)
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: 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: When Ray is at the News Producer's van, she grips and starts to push the videotape with her palm up, but in the next close-up it's the opposite, she's pushing it with her palm down. (00:44:00)
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)
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: In the scene where Ray, Robbie and Rachel are going outside of the house to find the remains of the airplane, as Ray is putting Rachel in the car, in the right hand side of the shot you see Robbie walking towards the wreckage, and he puts his hands over his head. In the next shot, it's a close up of him and his hands are at his sides, where he then proceeds to put his hands in the position they were in in the previous shot. (00:45:45)
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: When Ray parks the van at the side of the road, the van is about 60-70 ft away from the tall wood post, at the road intersection up ahead. However, in the first shot of the military vehicles approaching, the van is parked directly in front of that wood post at the intersection, and this also occurs in the shot as Ray shouts, "Robbie! You wanna go in that direction?" (00:47:40 - 00:49:00)
Continuity mistake: When they stop so the kids can go to the toilet, in a long shot of Ray trying to persuade Rachel not to go too far, we see him start to open the boot (trunk) of the car. When it cuts to a closeup he's not. (00:47:50)
Continuity mistake: When the van is parked at the side of the road, as Rachel heads off Ray shouts, "Every time you guys don't listen to me I'm telling your mother..." In the next shot the van is seen quite a distance away as Rachel reaches the shoreline. After seeing the floating bodies Ray picks her up, turns around and as he begins to run the van is seen, only this time is much, much closer. Ray never moved the van, as shown in the following shots. (00:48:00)
Continuity mistake: When Robbie shouts at the soldiers driving by, puddle water splashes all over him, and the pattern and/or amount of water all over Robbie's jacket varies shot to shot. For example: when Ray shouts, "You're in charge now, you tell me..." there are many water spots on both sides of Robbie's jacket, but two shots later as Robbie shouts, "We get back at them!" only his left side is wet. Quick drying. (00:49:10)
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)
Continuity mistake: In the countryside, after Rachel witnessed many dead bodies float down the river, and they head back to the van, an Army Convoy rolls by. While rolling by, Robbie starts jumping up and down waving his arms. His pants creep down enough to see his butt crack and reveal his red underwear. The next moment, picking up at a different angle, his backside is now covered by his grey underwear. (00:49:25)
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)
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.