Crew or equipment visible: When Mulder breaks the chair in the motel room during his version of the story a hand is seen placing a stake onto the remnants of the chair. This is possibly deliberate because it adds to the comedic value of the two-story theme for this particular episode, with Mulder and Scully arguing about minute details.
Factual errors: While doing an autopsy, Scully is weighing organs and incorrectly calls the small intestine the large intestine.
Revealing mistakes: During the autopsy of the first patient, when the camera shows a close-up on the patient's neck where the bite marks are, you can see the actor's pulse right above them.
Continuity: During the first autopsy that Scully performs, we see the victim's body with the chest already open. Scully later says that she'll "begin with the Y-incision", but this has clearly already been done.