Deliberate mistake: When Commander Taggert and Lt. Madison are about to press the blue button to stop the ship exploding, Lt. Madison's tunic is mostly zipped up. A few seconds later it is suddenly mostly unzipped. Those of you who have access to the DVD of the film can see that there was a deleted scene which was originally intended to go in here where she unzips her tunic to distract the aliens.
Deliberate mistake: When Austin and Felicity cross the street when they are shopping, you can see green hills behind them, very much like Hollywood and not 1969 London. (Deliberate reference to an earlier joke)
Deliberate mistake: Even though Hamm is flicking through the channels at the same speed, the side with Al's advert on it is visible for longer.
Deliberate mistake: While Hector is in the water with the crocodile, he yells to the female officer to start the helicopter by turning the key. It takes a lot more than just the key to start a Bell Jet Ranger helicopter. Fuel shutoff valve, throttle, key, and the main rotor doesn't start rotating until the throttle on the collective stick is turned to a specific position.
Deliberate mistake: Right after Stevie and Marla attempt sex in his snowplow, the camera cuts to Marla buttoning up 3 buttons on her sweater which takes less than 8 seconds. However, in that same amount of time, making little or no sound to support his actions, Stevie manages to put on his shirt, zip up his pants, pull back up his coveralls, re-Velcro them closed perfectly all the way up to his chest, pull up and reposition the suspenders to the coveralls and be sitting calmly with both hands on his thighs when the camera cuts back to him. In less than 8 seconds, all silently and not moving the cab to the snowplow at all, no way. (00:32:20)
Deliberate mistake: The shot of Ardeth and his companions watching Rick leave Hamunuptra at the start of the film is reused again when Rick, Evie, Jonathan and the Warden re-enter Hamunuptra, although the reused shot has a blue filter over it. Ardeth even says exactly the same line he says in the original shot, but it is subtitled differently.
Deliberate mistake: When the father hands the porno mags to his son, if you look real carefully, you can see the son hide one of the covers from the camera with his hand. (00:25:39)
Deliberate mistake: Rebecca's 30-06 has no scope and no open sights. So having gotten it for her birthday several years earlier, it's still pretty unprepared to fire.
Deliberate mistake: In the scene where Alicia Silverstone and Dave Foley are in the porno store, they cut to a guy standing behind a counter. All of the explicit shots of the videotape covers on the countertop are blurred out.
Deliberate mistake: In the short scene where Jip is driving his car and talking to the older man in the passenger seat, the car is driving down Marlborough Road. The camera cuts between Jip and the man several times, one shot of Jip shows a building where some tennis courts are, another shows Marlborough Road School and the last one is going past some houses. In the last one he should be going past a shop called Bedybuys.
Deliberate mistake: The song "If you give me your attention" (from "Princess Ida") and the Mikado's song (from "The Mikado," of course) are performed with their central verses omitted. This was no doubt to save time and, in the case of the Mikado's song, to avoid inclusion of the line "is blacked like a ni**er" (altered in 1948 to "is painted with vigour").
Deliberate mistake: The German army is using American Sherman tanks, presumably easier to get for filming.
Deliberate mistake: When Big Gay Al sings his song, he sits at the edge of the stage. In a wideshot the soldiers are just watching, yet in a closeup they're all waving lighters. (00:59:25)
Deliberate mistake: When Dooley looks at the photo of himself, Jerry Lee and his wife Tracey, it's obviously a still shot from the first movie, as the angle the picture is taken from was obviously not taken by anyone else as they were the only two in the apartment with Jerry Lee at the time. (01:11:40)