Will Smith's character accidentally drives the UFO in reverse, then apologises and turns the instructions upside down. Great for Rule of Funny, but after thought you realize that now he's sacrificed left and right being on the correct sides for having front and back being correct.
At that point he'd know about it beforehand and can compensate on the fly. The text being upside-down would be a reminder "Hey, left and right are flipped now."
Actually this is sort of handwaved: when it's mentioned that the alien ship has changed course toward them, one of the military personnel tells Jeff Goldblum's ex not to worry since they're well underground. Then she reminds him about the hundreds of refugees on the surface outside the base. Cue Oh Crap moment.
The novel explains this: the alien attackers fired on NORAD continuously until the mountain was blown away, then they took out the base inside.
David tells Marty to get his mother out of town. Marty tells her to go to her sister in Atlanta...which is later mentioned as one of the cities taken out in the second wave of attacks.
Patricia Whitmore doesn't actually know her mother is dead. Someone, likely her father, would have to have dropped that on her later.
The Roswell crash was a ship belonging to the movie's aliens. It was held in Area 51 since the 50s. Part of the conspiracy theory mythology involving the supposed aliens and alien technology at Area 51 (and possibly mentioned in the movie, it's been a while) is that a lot of modern technology was reverse-engineered from the alien technology in the ship. Put all this together, and it no longer seems quite so unreasonable that David could hack into the alien mothership with his laptop; it was probably reverse-engineered from the alien ship in the first place.
I believe that this was explicitly given as a reason in the novelization.
Lack of security in their computer network is also somewhat understandable — they communicate telepathically, and are "like locusts". Very probably they have some version of a Hive Mind, which would preclude the need for that much security, as no-one would sabotage themselves. They just didn't consider that one of their victims would find a way in and use it against them.
Similarly, this shows that they have been doing Recon on the Human Race and America in particular for some time... this is presumably why they attacked when they did. Remember the comment that 40% of American soldiers are either on shore leave for the holidays or massed in Washington for the 4th of July parade? The Aliens knew that they were both unprepared and there would be a significant portion in DC so they could take a large part of their military leadership in one fell swoop. This is also why they targeted the White House.
It could also serve to explain why Captain Hiller can fly the alien ship. Every sub-sequent fighter-jet which came after the ship crashed was probably based on its technology in some way, shape or form.
Why are the destroyers the size of a city when the actual weapon takes up a few blocks at most? If they made the destroyers smaller they could build more and attack more cities at once. However, having a large surface just above the city being destroyed channels the energy from the central weapon, making the weapon many times more efficient. — Anemone
It's not only a large surface, it is a large surface protected by a Force Field.
Also, their entire civilization is contained solely in the mothership and those destroyers. A destroyer isn't a military vehicle; it's an entire military base and the surrounding town to boot. They don't just fly the things; that's their home. It makes sense that it would be built for all the size they can afford to give it, as this is where they spend their entire lives.
Not to mention, we hardly see all of the weapon, just the "muzzle" so to speak. There could be dozens of power plants and subsystems for the thing, heat sinks, plus all the space needed to house the crew that works on it, repair facilities, aircraft bays, maintenance for the aircraft, the freaking ENGINES that allow a thing the size of a city to hover indefinitely without running out of fuel or flattening everything below it...
Russell hits the wrong button while preparing to fly, almost firing one of his missiles while still on the ground. Later, his final missile, also the last one the Americans have, fails to fire, possibly because he messed up the firing sequence before. Had he not messed up the setting earlier, the missile might have hit like the President's and then boom, goodbye Area 51.
Not really, since he'd merely armed the missile while on the ground. The error with the weapon during the final firefight is specifically shown on-screen as a 'clamp malfunction', which couldn't be induced by simply arming the explosive.
On the Headscratchers page, it's pondered what the Invaders would do if we didn't have satellites for them to hack...but then again, there is the possibility humanity reverse engineered a lot of our tech from the Roswell crash. What if that's their MO? They knew we would have satellites they could hack because our satellites are based on their technology and run on their OS...which comes back to bite them in the whatever-they-have-for-an-ass when man, in turn, uses the same OS to hack their shields. In fact even if we discount the possibility a lot or even most of our modern society is built on pirated Invader technology, what if they just attack societies which have reached a certain level of technology, so they can use their satellites and whatever else they can scavenge. Consider for a moment the plot of the game MDK, which puts forth the idea that the aliens (quite similar in many ways to the Invaders) "harvest" the technological riches of a planet as much as its natural resources. The quote, paraphrased, from the creators is that they see our cities as a resource, so they built these massive mining ships to harvest them. The ID 4 aliens could be the exact same way. In fact these two theories are not mutually exclusive.
It is also possible that the aliens just co-opted the communications systems to save themselves some time in setting up. For all we know, a lack of said satellites might've just delayed the final attack by hours. Hard to say, really.
Similar to the above, some detractors of the film call out the ridiculousness of David's Mac laptop being able to hack into the alien mothership. This seems to disregard the fact that Area 51 has had example of one of the alien craft for decades to study, and no doubt did the same thing David ended up doing during his demonstration of the deflector shields: Hook it up to one of their own computers and try to analyze or control its systems. The entire reason David is able to hack the mothership in the climax is because the Area 51 scientists already had, and he was just using their research to devise and upload the virus.
Of all the cities in America, Las Vegas could very well have been spared. According to the maps at the end, Area 51 is not too far from Vegas. The aliens were able to quickly redeploy a ship, as the main page points out, after picking up a sudden spike in communications from a previously-silent patch of desert. This means the destroyer was likely on its way to drop the Hammer on Vegas, or on its way from.
And to add onto this, it could very well have been the same ship that nuked LA. Payback is a bitch, ain't it?