When Hathaway first goes into the laser lab and exchanges conversation with Kent and the others, you can see the shadow of the overhead mic swinging back and forth. The shadow appears faintly on the far wall.
Kent picks up popcorn off the floor and eats a piece, exclaiming, "Popcorn!" In addition to it very obviously appearing to be popcorn, people with braces (as Kent has) are not allowed to eat popcorn as it can damage the wiring. Kent, being an obvious stickler for rules, would know this.
When Chris is excited about possibly figuring out the solution to the laser's power problem in the break room, he leans into the snack machine and jumps up and kicks the freezer door, knocking off the shelf bar. in the next shot the shelf bar is back in place.
During the party scene, Chris first puts a blue lei around Mitch's neck and has a red lei in his hand. In the next shot, Mitch is wearing the red lei and Chris is holding the blue lei. Then in the next shot, Mitch is wearing the blue one and Chris is once again holding the red one.
When Chris is taking his final exam, he is wearing a yellow t-shirt with a blue-striped shirt tied around his waist. During the exam, Chris is seen wearing this shirt over the yellow t-shirt. When he is done, he walks out and the striped shirt is around his waist. When he enters the lab, the striped shirt is over the t-shirt again.
When Mitch is leaving his dorm room to go to class and encounters the ice made by ICK he is carrying a text book. He trips and falls, and the book disappears. Mitch makes no effort to retrieve or look for the book so that he can proceed to class.
In the scene where Kent goes into Dr. Hathaways house and sees the popcorn wrapped in foil he touches it the foil is rounded and full but when the laser hits it its much flatter then begins to pop and bulges.
Before the shot is fired on Dr. Hathaway's house, the aircraft used to deploy the laser is shown as three different planes. On the base, it is a black SR71 Blackbird, on the approach it is the spaceship from the opening of the movie, and on the closeup shots it is a camouflage fighter jet.
When Chris makes a fake coin from the liquid nitrogen container, he inserts it into the drink machine. It's at least large enough for a quarter, which is what the machine display says it will take (along with dimes and nickels). The machine says drinks are 20 cents, but when he presses the button for his drink, we don't hear the change coming out. Also, he then turns to the candy bar machine and never inserts a coin but can get a candy bar.
The laser that Knight finally gets to work supposedly works at
600 nanometers. Unfortunately, that would make it an orange laser, not a violet one as shown in the film (which would have a wavelength closer to 400 nanometers).
During the laser test, the range controller clears the B-1 to Flight Level Six-Five-Zero, which is 65,000 feet. The service ceiling, or, highest altitude the B-1 can attain, is around 40,000 feet, and has never flown above that altitude. However, 65,000 feet was an erroneous altitude, because the outside shot of the aircraft shows that it is flying at low level, just over the mountains and the desert floor is visible. This would not have been the case at 65,000 feet.
When Kent tattles to Dr. Hathaway about Chris and Mitch being at a party with girls, Kent stands in front of a teleprompter on a television studio set, while Dr. Hathaway is taping an episode of "Everything". Assuming these are through-the-lens teleprompters for television cameras (so that the on-air performer, in this case Dr. Hathaway, can both read the text and look directly into the camera lens and talk to the TV audience), the shot of the monitor showing Hathaway looking around for his script and yelling, "Kent" would not be possible. Kent would be blocking the camera as well, and so all you'd see on that monitor would be black (i.e., Kent's body blocking the camera). Unless, of course, the monitor was showing the output of another camera, but in all likelihood, the in-studio monitor as depicted would be the program monitor, showing the "hot" camera, and Hathaway was trying to look into that very camera and read his teleprompter script. Not only that, but in reality Kent would have never been allowed on a presumably closed set to block a camera/teleprompter in the first place.
The false IDs used by Chris and Mitch to enter March AFB are shown as DD Form 2A - which is an identification card for military personnel (one of them is shown to have a pay grade of "O-1" - for the Air Force, that's a Second Lieutenant.
Chris tells the gate guard that "Someday you might be in the private sector, too" - but if they're covered as military officers, they're not "private sector" personnel
With a DD2A, they'd also have had an easier time getting through the gate in uniform, rather than in civvies.
The coordinates entered into the computer to redirect the laser at the end of the movie actually point to a place in China near Dongkan Town (34D 10M 15.21S North, 119D 7M.) However, if treated as West longitude, the location is near Oxnard, California (slightly north of Point Mugu NAS).
They say the ice in the hall would sublimate directly to gas. If that would be true, it would be impossible to skate or glide on this kind of ice as the building of water is essential for gliding/skidding on ice. However, Ick never reveals what the substance is, and it's possible his mystery substance can both sublime and provide for skating.
They speak of increasing laser output by "an order of magnitude" to 6 megawatts. While they are currently at 5 MW, the increase referred to an earlier model which was presumably operating at around .6 MW.
When Jordan brings Chris and Mitch the fake IDs to get into the military base she first gives the wrong one to each of them, then switches as Chris Knight (Val Kilmer) is looking at his he flips it over to another picture of him, but the name on the top is that of Mitch Taylor (Gabriel Jarret). Jordan gave them the wrong badges because she accidentally mixed up which picture went on which badge.
In the scene where Chris is cutting dry ice coins for the vending machine, a fire extinguisher on the wall behind Mitch is seen in some shots but not others. However, a student can been seen removing the extinguisher, but it happens very quickly.
When the students are trying to break into the military
computer, Laslo comments, "Oh, we'll get in; it's just going to take six hours." A six letter string using only the 26 letters of the alphabet (and no numbers or case changes) has approximately 308.9 million possible combinations. There are about 31 million seconds in one year. That means that trying every combination once every five seconds would give Laslo the chance to try about 6.3 million combinations per year. Given that there are 300 million possible combinations, it would take approximately 47 years for Laslo to try every possible combination.
When Mitch discovers the secret passage to Laslo's hideout, it is after Laslo went into it and disappeared. As Mitch opened the panel to find it, a car is there to take him to it. Problem is, there's only one track and one car. Also, the amount of time it takes the car to travel to it is so long that even if Laslo sent the car back (which would be the only reason the car would be there after he went down there and a highly unlikely thing if he's technically the only one who knows about it or is the only one who goes there) it wouldn't return so fast as to be there for Mitch to see and get into, considering he found the passage only mere minutes after Laslo went that way.
When the front door of Jerry's house is burst open from the popcorn joke, popcorn is already visible on the porch next to the door. In this scene, new popcorn falls into the exact same position (indicating previous takes).
When Mitch finally cracks and calls his parents to come home, Kent and his friends decide to record the call as a joke. Later when the call is played over the PA in the cafeteria, it includes the first 10-20 seconds of the call, which would not have been included as that was when Kent and them went to start recording it.