Continuity mistake: When Harry is going to A.J's room, he bangs on the door with a golf club. You can see the white fan fall to the ground. Then when Harry walks in the fan is back on the ledge. (00:12:15)
Other mistake: There's grass on the edge of the cliff when the Armadillo finally lands - look just under the Armadillo while the astronaut's pulling himself up the road. Considering this is an airless, waterless asteroid, it doesn't really fit...
Factual error: Steve Buscemi opens fire with what appears to be a variant of a General Electric GAU-8/A Gatling Gun which has an effective recoil of approximately 10,000 pounds-force (45 kN) per round discharge of armor-piercing ammunition. The vehicle carrying the cannon does not buck upwards or backwards at all. When the significantly decreased gravity of the asteroid is factored into consideration, at the very least the vehicle should have rocketed backwards or flipped upside down! Additionally, the barrels would have been cold-soaked to something close to -273° Celcius when the spinning asteroid was pointed away from the Sun, the heat generated by the rapid fire rate (combined with the friction of the slugs passing through the barrels) would certainly cause the frigid barrels to shatter like glass. And why would NASA fit a Gatling-Gun on a drilling platform in the first place? There are chemical lasers that would be much more effective and certainly lighter in weight should a mission call for the option of blasting one's way through an obstacle.
Continuity mistake: When A.J.'s Armadillo needs to break out of the space shuttle, the crew shoot up the walls of the hull of the ship. But then when they drive the Armadillo through the hull, the ship breaks apart at the seams rather than at the bullet holes. (01:32:20)
Continuity mistake: There are two crews boarding their spacecraft, called "Freedom" and "Independence." They are mounting a ramp and when on top, there is a crew member who directs the two different crews toward their ship: "Independence on the left, Freedom on the right." We even see the ships (in their initial launch phases) right next to each other - you can see one right next to the other as they launch. Then, once they're airborne, we see a long shot of the two bright engines, miles apart. (01:03:35 - 01:06:25)
Revealing mistake: In the New York meteor shower scene, most of the cars blow up without having been hit at all.
Factual error: During the teams physicals, the doctor mentions Ketamine. While Ketamine is a sedative it is not strictly an animal sedative. Ketamine is a mid-range sedative that doesn't even put the patient to sleep. I use it in the ER I work in almost every day to set broken bones on children. The good doctor was a bit overly dramatic when describing the drug.
Revealing mistake: The two ships that the astronauts and drillers use are supposed to be new "X-71" military vessels that were top secret - and just happen to bear a resemblance to the actual NASA shuttle orbiter. But, when they are launched, it's modified shuttle footage, and in at least one of the long shots they didn't add the X-71 overlay. The key distinction is the two top-mounted booster rockets on the shuttle that the real shuttle doesn't have. There are also a few shots of the firing engine on the mission control screen that is the unmodified shuttle and not the X-71 configuration or paint scheme. This occurs later as well during the MIR docking sequence when mission control shows footage of a real docking arm that looks nothing like the soft-docking arms of the station.
Continuity mistake: The first meteor is a direct hit on a street vendor in NY where the dog is chewing on one of his Godzilla figurines. To start with he's chewing a small Godzilla, then it cuts to the vendor tugging on it, then it cuts back and its suddenly a much larger one.
Continuity mistake: When they are discussing the plan to jump the armadillo, A.J. Frost's left ear goes from having a clear earpiece over it in profile, to having a yellow earplug in from the front.
Continuity mistake: Even though the second shuttle takes off later then the first, they are neck and neck when they clear the atmosphere.
Continuity mistake: During the scene where AJ is playing with cookies on Grace's stomach on the last night before the launch, the car behind them alternately appears and disappears as well as moves around. Best example - wide shot shows them well behind the car, then in a shot of Grace her head's right by the tyre. (00:51:00)
Continuity mistake: When Harry goes into A.J.'s room, he gets mad and sticks the end of the golf club at A.J.'s throat, with the tip of the club to the right. The next scene, the tip of the club is facing to the left. (00:13:30)
Factual error: Setting the thrusters on in the "Armadillo" while it is spinning, would cause the vehicle to spin either out to space, or to crash into the ground.
Visible crew/equipment: When on the asteroid, Rockhound starts to play with the gun. To stop him Harry goes over and pushes him off and they both go flying through the air. Just before they hit the floor you can see the zip wire they flew across on.
Continuity mistake: When they are in the shuttle trying to disarm the bomb, it is the pilot that is telling the munitions specialist, the guy that is supposed to know everything about the bomb, which wires to cut. (01:44:35)
Other mistake: In the sequence where they are disarming the bomb. When the clock gets down to 00:24, the picture is reversed (the split second shot when they pull the clock out of the bomb). (01:48:35)
Continuity mistake: When the two shuttles are trying to land on the asteroid, the shuttle Independence crashes, and it is shown crashing upside down and sliding across the surface of the asteroid. Later, having survived the crash, AJ and the Russian are outside the shuttle and it shows it sitting on it's belly, right-side up, with the words United States visible. How did the shuttle suddenly flip itself upright when moments before it was upside down?
Continuity mistake: Just after Harry says "prepare the world for bad news," air raid sirens start. There's a panning down shot of some air raid speakers next to a Coca-Cola ad, with a truck parked in front of it. The angle then changes to a shot down the street (same street, same speakers, same Coca-Cola ad), except the truck isn't in front of the ad any more. (01:56:10)
Continuity mistake: When the crews climb onto the buses to travel out to the shuttles, none of them are wearing sunglasses. When they exit the bus those that are seen are wearing sunglasses. However, the following 'slow mo' shot of everyone shows they aren't again. (01:03:00)
Answer: If you look at the deleted scenes on the special edition DVD, you will see a deleted scene in which A.J. asks what they needed a gun for, and Max explains that it's for debris elimination, in order to take out small rocks in the way.