Continuity mistake: In the beginning of the film, when the first tripod began to rise from the earth, there is a cut showing two men standing on top of their scissor lift, which is swaying dangerously back and forth because of the earthquake the tripod is causing. In the very next cut, showing the entire area, the scissor lift is very apparent in the foreground, not only is it not swaying (while everything else in the area is shaking dangerously) , but one of the men is suddenly in a kneeling position, whereas he had just been standing up. (00:22:40)
Continuity mistake: When Ray is outside at the beginning of the movie during the lightning storm, he says to himself that the wind is moving in the direction of the storm. Yet when he calls to Rachel to come outside and take a look, her hair is blowing away from her face, in the opposite direction it should be blowing.
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)
Visible crew/equipment: After the ferry disaster, and close call with the car, the trio are underwater swimming up towards the surface. When Robbie turns his head to the side the regulator in his mouth is visible. (01:03:45)
Revealing mistake: When the tripod is tipping the ferry over, people fall over the rail and cars slide toward it. You can see the cars stop short of the rail so they don't smash into stuntmen, even though they are jammed against the rail in the next shot.
Continuity mistake: In the interior shot, the Hudson Ferry captain walks toward and then stands at the doorway (seen clearly when he cast off earlier), as he stares at the growing whirlpool. However, in the next exterior shot he is standing at an open porthole window, staring at the water. (01:02:45)
Continuity mistake: When Ray, Robbie and Rachel arrive at Mary Ann's house, as the kids shout for their mom and Tim, the door to the basement under the stairs, as well as the den further up (beside the kitchen), are all perfectly visible. Later, after the house is destroyed, when Ray walks through the basement doorway, the walls, moldings, and so on are entirely different, and now there is an extra wall (perpendicular to the camera) to the left (viewer's) where there was none before. (00:35:10 - 00:41:40)
Revealing mistake: At one point when Cruise is driving through the county in the van, you can see that the van is actually in "Park." You can notice this because the column mounted transmission lever is all the way up.
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 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)
Factual error: When they're near the ferry, the fiery train zooms by. Passenger trains along the Hudson River run along the east bank of the Hudson, not the west bank, where the ferry was departing. (00:56:55)
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)
Revealing mistake: After the ferry scene, CGI clothes fall off from the sky, but manage to go through the branches of the trees, without becoming entangled in them, or without the branches affecting the falling path of the fabric.
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)
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 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)
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: 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)
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.