Unrequited - S4-E16
Deliberate mistake: When Mulder is about to go into the metal detector he puts his gun on the counter. He goes through the metal detector on his cell phone but his keys set it off, when he steps through again it doesn't go off. No cell phones can go through metal detectors without setting it off.
Via Negativa - S8-E7
Continuity mistake: At The Beginning of the scene, there is only what seems to be a remote control on the left-hand side of the TV. Five seconds later, a couple of VHS tape cases appear where the remote control was before. (00:08:46)
Continuity mistake: Although the subtitle states 08:02 PM at the morgue, the clock in the room shows 08:40. (00:12:40)
Soft Light - S2-E23
Plot hole: If the shadow acts like a black hole and tears apart all atoms that enter it why does it only seem to affect people (but also their clothing and anything else on them)? Surely Dr Banton would be destroying everything around him.
Factual error: At The Beginning of the scene, XO Johanson is looking through the periscope. However, it is nonsense, because the submarine was in silent running, and the Japanese destroyer was circling it, so the sub must have cruised in deep submersion. The water depth there was approx. 270 meters, as the French salvage team claims at The Beginning of the episode. There is rarely any significant light beyond 200 meters. Looking through the periscope in these conditions would be completely useless. (00:32:25)
Revealing mistake: At the very beginning of the scene when the bomb is shown, in the lower part of the screen a moving red spot appears which probably comes from the reflection of the bomb's red diode in the camera. (00:36:50)
Other mistake: Eve 6 says that ordinary human beings have 46 chromosomes, while the Adams and the Eves have 56 chromosomes. This makes 10 additional chromosomes. However, Eve 6 also mentions that they have additional chromosomes number 4, 5, 12, 16, and 22 - only 5 additional chromosomes. (00:22:40)
Anasazi (1) - S2-E25
Character mistake: Mulder asks Scully about "the fourth commandment about obeying the Sabbath, the part when God made heaven and Earth " However, Scully as a Catholic should know that this is the third commandment for Catholics. This is also an indirect indication that Mulder was raised in Jewish religion. (00:09:25)
Factual error: It's quite strange that only after Mulder looks into the ashtray he realizes that the Smoking Man has been in the car. Mulder should smell the scent of cigarettes immediately after entering the car, especially if he is a non-smoker. (00:36:05)
Factual error: The crew are supposed to be French. However at The Beginning of the episode we can hear them talking in French with a Canadian accent.
Plot hole: Mulder is pulled from the escape hole previously dug by the hybrid aliens to escape the train car. When they pull him out they show alien bodies remaining in the hole. Mulder never revisits the train car despite knowing there's a high probability of finding hard evidence, in this case some actual bodies. There's no way Mulder would just assume there's no evidence remaining there and not go have a look for himself. Same goes for Cancerman who thought he had destroyed the evidence.
Deliberate mistake: When the unconscious, naked Asian doctor is removed from the warming tub, his hands are clasped over his crotch to protect his modesty, despite the fact that he shouldn't be able to do that, since he isn't conscious and his previously frozen limbs are now pliable and relaxed due to the heat.
Factual error: This episode begins with Scully doing a voice-over about the incredible nature of our universe. The scene is very impressive visually, and would have been from the aspect of her oral presentation had it not been for a major error in the script. She says (more than once) that the universe is made up of matter and gas. This should have been "matter and energy". Gas is a form of matter.
Plot hole: On the phone, Scully asks for the report on her sister to be sent to the San Diego F.B.I field office (since she is visiting her brother and his wife and so is not in Washington), but the package is sent to her brother's home. Even if 'Danny,' whom she asks for the file, decided to be helpful and send it directly to her, her brother's address is not likely to be the one in her personal file, plus she didn't mention that that's where she'd be staying.
Factual error: When Scully is performing the autopsy on Leonard Betts' severed head, she states "As remains are incomplete, all observations refer to a decapitated head." This phrasing is redundant. The proper terminology would be "disembodied head." As a medical doctor, Scully should know this.
One Breath - S2-E8
Other mistake: At The Beginning of the episode, a gravestone for Scully is shown. However, it seems as if less than a week passes between Scully's mother and sister deciding to let her go and Scully waking up. It's unlikely that the gravestone business would have the stone ready in such a short time.
Revealing mistake: When Edward aims the rifle out of the window, you can see the cap is still on the scope. (00:40:22)
Revelations - S3-E11
Continuity mistake: Kevin's mother uses her car to hit the 'devil' and shatters the bottom right of the windscreen. The following shots, including the crash into the ditch, show the car with the windscreen undamaged. (00:28:59 - 00:29:22)
Visible crew/equipment: After talking to the scientist about his car, Mulder and Scully are talking in front of a door. In the glass of the door is a reflection of a third person.
Answer: As he stated many times throughout the series, Mulder needed Scully to be sober and skeptical. Whenever Scully's skepticism wavered and she started questioning her own rationality, Mulder would try to restore her sense of skepticism, because he needed her to be clear-thinking.
Charles Austin Miller