Continuity mistake: When Mulder is talking to Dana's mom about finding her cross (necklace) in the trunk, Scully's mom says that she gave it to her on her 15th birthday, but in Season 5 in Christmas Carol, Scully has a flashback to receiving it at Christmas.
Continuity mistake: During the previous episode, Skinner receives a nasty gash on his head courtesy of Billy Miles. This episode takes place 48 hours later and there is no indication whatsoever that Skinner received such an injury (the wound is visible once again in the next episode).
Revealing mistake: It seems that in the first shot the doctor injects something into the vaccine vial rather than extracting from it since the syringe plunger is moving inwards instead of outwards. In the next shot one can see that the syringe is being filled with the vaccine, however the arrangement of doctor's fingers is suitable for injection, which strongly suggests that the scene is played backwards.
Revealing mistake: The establishing shot of Boston General Hospital at the end of the episode shows the same building that was used for the BYU Medical Center in Provo, Utah eight episodes earlier in "Roadrunners" (note the vegetation under the part of the building that extends over the curb and the lettering on the building that says "Entry A"). The actual building is presumably in Kansas since the Kansas state flag was visible outside the building in the aforementioned episode.
Continuity mistake: In this episode the character of Billy Underwood was played by twin brothers Kyle and Ryan Pepi. If you pay attention you can tell when one twin was being used instead of the other as one of them has two freckles on his neck and another two by his right eye whereas the other does not.
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Continuity mistake: When the Lone Gunmen speak with Susanne Modeski, in the background there are some cars parked in front of the Stadium Inn. However, just seconds after, when the telephone rings, there is a white truck parked at the entrance. Later on, the cars are back again.