Revealing mistake: As the smoking man sits on a bench after finding out his story was rewritten, there are mountains visible in the background. Atop one of the buildings in the background is a Canadian flag waving in the wind. This show was shot in Vancouver, but is set in Washington D.C. There are no tall mountains that close to D.C.
Visible crew/equipment: When Mulder and Scully enter the abandoned mine through a doorway, the viewer can see at right in the distance a number of people sitting on a wall, and one crouching in front of the wall with what looks like a dog. They are sitting still, but there is still movement there. (00:16:00)
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.
Continuity mistake: Clyde Bruckman commits suicide by placing a plastic bag over his head until he asphyxiates. However, when Mulder and Scully first discover him you can see in the shot just before Scully holds his hands that there is no plastic bag around his head (may only be visible in widescreen format). (00:42:55)
Visible crew/equipment: A mic boom operator is visible on the left hand side of the screen wearing a white t-shirt, beige shorts, and sneakers. He's carrying the boom mic and following Mulder and Scully. You can see him when Mulder and Scully arrive at the "Church of God with Signs and Wonders" for the first time...when Mulder delivers the line "That's funny, I knew a couple Catholic schoolgirls who were experts at it." (00:08:20)
Continuity mistake: When Mulder is in the tram car ascending the mountain to rescue Scully, he pulls all of the things out of under the top left-hand seat, but when the scene moves to Krycek in the booth, you don't see any of the things that Mulder threw out. (00:19:40 - 00:20:50)
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)
Continuity mistake: When Tooms is in the cell right at The End, we see when his food is being brought over to him on the tray that there is a drink and a bun, when the shot changes to the other side of the door the drink and bun have swapped sides. When the shot changes back to the other side of the door, the tray is now the other way around and then when the shot goes back to the other side the whole tray has now disappeared. (00:39:40)
Continuity mistake: 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. (00:08:35 - 00:09:30)