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.
At around the 3 minute mark, while Mulder is kicking the garbage can, Scully's hands switch from being clasped together to her arms being folded between shots.
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.
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 for this particular episode.
During the final, daylight scene in the trailer park at around 43.50, snow-capped mountains can be seen in the distant background. Vancouver yes, Chaney, Texas, no.
While doing both autopsies, Scully is weighing organs and incorrectly calls the small intestine the large intestine.
At 25m 40s, they talk about 247 matches being dropped in Rain Man (1988). In that film, there were 246 toothpicks dropped.