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.
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.
Continuity mistake: Mulder says that Scully was abducted and missing for four weeks. In "Duane Barry," the date stamp is given as August, then in "3," when Mulder returns to the X-Files office he turns the calendar all the way to November. Since Scully is returned the next episode after this, she must have been missing for at least 3 months, not 1.
Continuity mistake: When the bank manager goes to open the door for the brinks truck containing the robbers, the clock on the wall reads 9:45. When the robbers are taking him back to the vault, a couple of shots of the skull-masked robber shows the clock reading about 9:43.
Continuity mistake: At the end of this episode when Mulder is talking to Marty through her jail cell, the position of their hands change. In one shot, Mulder's hand is covering hers, and in the next, each of their hands are gripping the cell bars. This goes back and forth a few times.
Continuity mistake: This is probably one of the most intriguing chess plays in the history of mankind. In the first move shown on the screen Gibson moves a white knight from e4 to f6. Then, in the close-up, the situation on the chessboard more or less corresponds to the arrangement of the figures previously shown on the screen. However, in the next shot, when we see the chessboard from the Russian perspective, the arrangement is completely different. Especially, there is no white knight on the chessboard at all. In the following shot the white rook suddenly appears on g8 (and is subsequently beaten by the black king), but in the next move Gibson again moves his white rook on g8, while the black king has miraculously returned to h8 and again beats the white rook. Moreover, Gibson could not play two white rooks, because later on we see that there is still at least one white rook on the chessboard.
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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.