Continuity mistake: During the scene in the mall when the girl is in the dressing room, she keeps putting on more layers of clothing. But as they are shooting different shots she will have a hot pink shirt on, then it is back on the hanger, and then back on again. It switches back and forth many times.
Factual error: At the end of the film, when Duchovny and Jones are on the ladder truck shooting the head and shoulders, they are yelling "go go go". They slide down the ladder and the next shot is the fire truck speeding away. Movie elapsed time 5 seconds. It is a physical impossibility for a fully extended ladder to be lowered, retracted and bedded in a span of less than 4-7 minutes. Even if the hydraulic lines are cut, the ladder would remain upright and not collapse/retract.
Continuity mistake: When Ira and Jones break into the secured area to get the specimens, they lay down in front of the barbed wire. Ida is wearing a black Colonel Uniform and Jones is wearing a Private Uniform. But when they get inside the facility Ida's uniform is clean and has no dust marks.
Revealing mistake: This film was shot in Page Arizona with is pretty far north in the "high country". When they first drive up to the meteor side, there is Saguaro cactus that are only a couple of feet high with huge arms and a neon greenish color. These only grow on the desert floor in Arizona and they certainly are not neon colored. Additionally, the arms don't start appearing until they are much taller as they grow them to balance themselves since they have a very shallow root system.
Add timeJohn Hozian
Factual error: The aliens are supposedly nitrogen-based, making them vulnerable to selenium in the same way humans are vulnerable to arsenic because we're carbon-based. However, nitrogen atoms don't have the valence real estate to form the long molecular chains critical to the DNA structure shared by terrestrial and alien life (when Ira first analyzes the sample, he finds DNA with 10 base pairs).
Continuity mistake: During many scenes of the movie, you can notice that Orlando Jones has no glass in his glasses at times, then does at other points. When they are in court, he puts his fingers through the frame to rubs his eyes. [Some people have said they're meant to be fake in order to make him look clever, but they've definitely got glass in some scenes.]
Continuity mistake: At the end of the film when David Duchovny and Orlando Jones go up the ladder to squirt the Head and Shoulders into 'the hole', Orlando Jones gets sucked up into the alien and when he is pulled out he is covered in slime from head to toe, (including his glasses), as is David Duchovny. However, in the next shot of them going down the ladder there is no slime on them at all. In the next shot when they are standing at the bottom, while the truck pulls away, neither of them have much slime on them, and Orlando Jones' glasses are completely clean. The amount of slime they are covered in changes throughout the next few shots.
Plot hole: Because humans have a layer of subcutaneous fat under the skin, the bug that travels under Harry's skin would have caused not only significant damage but an amazing amount of pain while moving around. There would have also been severe internal damage caused by the bug if it traveled into his colon where they ended up removing it from.
Revealing mistake: When Harry and Ira take the students to the cave to excavate the meteor, pay attention in the first wide-shot back in the cave where you can see smoke/vapor on the ground. You can see a small patch at the very bottom of the meteor where the smoke is being pumped onto the ground, likely by a concealed smoke-machine either inside of the prop meteor or just underneath the set. You can see the smoke sort-of "churning" unnaturally at the base of the meteor as it is pumped onto the ground, giving away the effect. And to avoid any corrections, yes they say that the area is being adapted by the microbial life the meteor brought to earth and developing a mini-atmosphere in the cave... but this is an unintentionally visible effect. The atmosphere/smoke/vapor wouldn't be all coming from one single tiny spot at the base of the meteor, and it wouldn't be billowing and churning like smoke being pumped from a smoke machine.