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: 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: When Ray speeds home in his car he passes Manny's shop and directly in front of the left garage door, beside the gas pump, is a large tree surrounded by bricks at the trunk base. Later, when Ray speaks with Manny the tree and surrounding bricks are gone. (00:04:15 - 00:19:00)
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 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)
Visible crew/equipment: When Robbie gets out of Tim's car there are two of the crew's chalk marks on the sidewalk - one long blue line in front of Ray and an orange arrow near the trunk of the car. They are gone when Rachel leaves the car. (00:05:00)
Continuity mistake: When Mary Ann carries Rachel's backpack into Ray's kitchen, she places the backpack down on the floor to her left, with her left hand and then steps away. However, in the next shot, she is still holding it and then places it down to her right, with her right hand. (00:06:00)
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: 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 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: 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: While in his bed Robbie watches TV, as he leans against the wall behind him. Yet, in the close-up of the TV when Ray clicks it off, Robbie is clearly reflected on the TV screen leaning near the window, which is a good few feet away from where he was in the previous shot. In the next shot he's back to where he was in the first shot. (00:08:30)
Continuity mistake: When Ray and Robbie are throwing the ball near the start, check the wall behind Robbie. In closer shots of him there's sunlight on the wall and the shadow of a nearby satellite dish is visible, but in wider shots the whole wall's in shadow. (00:09:00)
Revealing mistake: While tossing a baseball with his son, Ray calls him a "dick." If you look at his hand at that point, one can notice that his hand is closed in a fist, and he is not holding the baseball he is prepared to throw. (00:10:10)
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: 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)
Audio problem: The audio of the "SpongeBob SquarePants" episode on the T.V. doesn't match with the episode. (00:11:20)
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: When Rachel says Ray should get TiVo, she is looking at her hand. When it cuts she is suddenly resting her head on her hand. (00:11:55)
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)
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.