Continuity mistake: Just before the first alien tripod breaks free from the ground and cracks are appearing in the street, the front of the church cracks, and rocks away from its second half. In that shot, in the view from behind Ray, he places his right foot on the curb, while his left foot is on the road. In the next shot facing Ray, both his feet are on the curb.
Continuity mistake: When Ogilvy shouts Ray's name half a dozen times, Ray runs to him to quiet him down. The broken cellar window behind Ogilvy has shards of glass and red roots growing on it. Then in the next exterior shot, the shards of bloody glass are different size and shape, and the roots differ as well. (01:29:35)
Continuity mistake: In Ogilvy's cellar, in the close-up of Ray's hand, he holds it up to the red mist blowing through the two broken windows. In the next shot however, as Ray looks at his hand and realizes it's blood, the right window is suddenly blocked with a broken shutter and two pieces of wood. (01:29:15)
Audio problem: As Tom Cruise and the children walk towards the Athens NY ferry an older gentleman says, "We got it the worst, that's what I heard". If you look closely his lips do not match what he's saying. (00:56:30)
Audio problem: When Tom Cruise gets back to his house after the first encounter with a machine, Robbie's says "What happened?" but nothing is heard, then he says it again and is heard the second time.
Continuity mistake: After speaking with Robbie, when Ray runs across the street to speak with Manny some are entirely different cars from the previous shots and in one case a car jumps sideways from one lane to the next. Ray also runs beside the crosswalk in the close-up, but in the next shot he's much farther away, and the crowd differs. (00:18:50)
Audio problem: In Boston, just after the military destroyed the tripod, in the tunnel Ray holds Rachel and we hear him say, "I love you. It's ok. It'll be ok," but his lips really say, "I love you. It'll be ok. It'll be ok." (01:45:10)
Continuity mistake: The Hudson Ferry employee runs to the two soldiers and shouts, "Get that ramp up. Cast off. No more peopleĀ " In the next semi close-up of the crowd converging on the ferry, there is a large grey car on the dock at the base of the ramp. In the next shot that car is inexplicably gone. (00:59:45)
Continuity mistake: When the hole cracks the asphalt in the first shot, the crack makes its way to Santo's Florist cracking the side of the brick building. A few shots later, when the Portuguese restaurant windows shatter, as the camera pans toward the church Santo's is seen on the street corner and the massive damage to the building is gone. The long crack in the asphalt leading from the hole to the florist is gone too. (00:21:20)
Continuity mistake: Whilst Robbie is helping the people onto the ferry in the overhead shot, Ray holds Rachel beside a red car down below as they are both avidly watching Robbie's heroism. However, in the next close-up Ray is just now approaching the car, amongst a greater crowd of people. (01:02:20)
Factual error: As the Hudson-Athens ferry capsizes, Tom Cruise and his two children are almost crushed as a car slides off the ferry and quickly sinks, going straight to the bottom. We see that this sedan-style car has people in it, and that the windows are closed, so the passenger compartment has not yet flooded. It is still full of air. A sedan full of air would not quickly sink. It would briefly float on the surface, then slowly submerge as the passenger compartment floods.
Visible crew/equipment: When Tom Cruise comes back in off the street covered in ash, he slumps down by the refrigerator and very quickly the boom mic falls down into shot almost hitting Tom on the head.
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: In Boston, when Ray breaks off a piece of the dried up root from the statue, the piece he holds in the next shot is completely different. (01:41:35)
Continuity mistake: When Rachel runs to her mom, in Boston, in the side shot of Mary Ann she's already gone down the stairs and crossed the sidewalk towards her daughter, yet in the next shot it's Rachel who crosses the sidewalk towards her mom, who is still up at the center of the stairs. (01:47:05)
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: When Ray carries the Pennzoil box of groceries into Mary Ann's house, the goods change position between shots. (00:35:20)
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: In the scene when the intersection is rotating, the camera is aimed up towards the church and starts to pan down. Tom Cruise runs into the scene and gets behind a car. The movement of the church shows the intersection is turning counter-clockwise but as it pans to the ground, the intersection is turning clockwise.
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)
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.