Revealing mistake: While the computer is being infected with the virus, Peterson turns away from the screen to face Scully. Two seconds later, after the shot of Scully, the same shot is shown again but in reverse so that Peterson turns back to the screen. It's more obvious if you look at the weird way his eyes blink in the reversed shot. (00:39:50 - 00:41:32)
Revealing mistake: When the doctor examine Ronnie's "corpse" in the morgue, he pulls out the stake. It is quite obvious that he's struggling with a wooden stake that is placed on the actor's side, not through his chest. Moreover, the camera pans up and you can briefly see Ronnie's unharmed chest.
Revealing mistake: During the autopsy, right after Scully has the vision of her daughter, the body of Paula Koklos can be seen to move (watch her hands and neck). (00:24:05)
Revealing mistake: When Mulder is examining the residue on the mouth of the deceased man in The Beginning, the dead body's right eye twitches.
Revealing mistake: When Mulder finds the Japanese doctor dead on the floor of the train bathroom, the man's eyes are open. Just before the shot cuts to the girl and boy at the door, he closes his eyes.
Fresh Bones - S2-E15
Revealing mistake: While Mulder, Scully, and the groundskeeper are walking through Folkstone Municipal Cemetery, there is a partially hidden but still conspicuous sign that reads "Vancouver Cemetery." (00:18:45)
Sanguinarium - S4-E6
Revealing mistake: When Mulder and Scully enter the doctor's house, you can see over their shoulders that it is showering with rain outside, yet both of them are bone dry.
The Sixth Extinction (2) - S7-E1
Revealing mistake: When Scully enters the F.B.I. Building after returning from Africa, she isn't wearing her badge. She'd need it to walk around without being stopped by security. (00:35:50)
Revealing mistake: When Mulder and Scully are talking to Theresa at the diner she suddenly gets a nosebleed. If you look closely at her forehead/nose you can see the flesh coloured tube that pumps the blood. (00:33:20)
Via Negativa - S8-E7
Revealing mistake: When Doggett examines in the car the body of Agent James Leeds, the carotid artery of the victim is pulsating. (00:05:40)
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)
Revealing mistake: When Edward aims the rifle out of the window, you can see the cap is still on the scope. (00:40:22)
Revealing mistake: At The Beginning of the episode we see Duane being abducted and he begins to scream. Look at the "alien" on the left of the screen near to the top of the bed and you will see him move his head revealing where the suit ends and neck/head begins. (00:02:12)
Revealing mistake: When Mulder, Scully and the other agent are looking at the covered body of the dead prostitute if you look closely you can see her chest rising and falling. (00:17:22)
Revealing mistake: During the opening funeral scene if you look closely you can see the dead girl breathe. (00:02:28)
Revealing mistake: At The Beginning of the episode Darlene burns her hand trying to open the door, she then puts on an oven mitt and opens the door. If you look closely though she still uses her other naked hand as well as the covered hand to open the door and her naked hand is completely unhurt.
Revealing mistake: After Scully handcuffs Tooms to the bathtub there is a close up of his face and you can see the edge of his contact lens. (00:39:07)
Revealing mistake: At the very beginning of the episode, we see the reflection of the Moon in a puddle. The Moon looks exactly as we see it in the sky. The problem is that in the puddle we should see the mirror image of the Moon. (00:00:05)
Trust No 1 - S9-E6
Revealing mistake: When Agent Reyes drives up to Agent Dogget's parked car, he looks in his rearview mirror. Its background should be obviously a transparent windshield to which the mirror is fixed. However, when the camera shows the image seen in the rearview mirror, we see in the background something similar in appearance to a solid wall. (00:10:50)
Revealing mistake: When Scully makes a purchase at the grocery store, shortly before she scans the metal fragment, the clerk gives her the total of the sale, however, shortly after the camera captures the register, which displays ‘no sale'. (00:41:30)
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