Continuity: The first time Scully walks into Mulder's office, we see his "I want to Believe" poster in full view. Later on in the scene, we see papers stacked in front of it.

Factual error: In this episode, Mulder says that he's red/green colorblind. Colorblind people cannot be F.B.I. agents.

Visible crew/equipment: I don't know if this is visible in all formats, but on the newest UK release on DVD, watching in widescreen, it's amazingly obvious. When Mulder recovers from the vampire attack, he gets up from the floor and breaks up a chair to make a stake. There is a shot of his feet as he smashes the legs off and bends to pick up a piece. In the bottom left hand corner of the screen, you can see a crew member's hand carefully and quickly placing a piece of wood on top of the pile for Mulder to pick up.

Other: In the episode "Musings of a Cigarette Smoking Man," when CSM is writing his letter of resignation on his typewriter, you can see that the letters are already there - he is simply typing over them.

Factual error: Lewis Brady had a "Felon wanted by the FBI" file in 1929, but the FBI wasn't called the FBI until 1935; until then it was just the "Bureau of Investigation."

Factual error: Scully stops Mulder from performing CPR on Amy when, as a medical doctor, she should have known that it is possible to revive cold-water drowning victims long after the heart stops.

Revealing: 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. Submitted by darren-c

Continuity: When Scully goes to Mulder's room, thinking she has the marks, after he has checked her we see Scully's hair is behind her collar, but when the shot changes to her going to sit down, her hair is now hanging out.

Factual error: When Mulder and Scully are moving into the house, there is a shot of Mulder's watchface which reads that the date is the 7th. A few minutes later, Scully is walking around with a video camera, and narrating and she says the date is February the 24th.

Factual error: In the episode "Rain King" when Mulder and Scully first arrive in Kroner, Kansas, you can see mountains in the background. There are no mountains in Kansas, only valleys and hills.