The X-Files Season 3 Episode 23

About the episode's halfway point, Mulder is trying to call Scully from a cell phone in his car. The tones heard imply he dials only seven numbers. In
1992 the telecommunication industry began to introduce "overlay plans" as a means to introduce new area codes. In this model, one fixed geographic area would concurrently have multiple valid area codes throughout. Overlay plans introduced a new inconvenience: mandatory 10-digit dialing (i.e., the area code must be included), even for local calls, in the affected area. This would be particularly true of cell phones and calls in and around large cities, like Washington D.C.

Principal Setting:
Braddock Heights, Maryland; Washington, D.C.

Scully uses the name "The Cancer Man" for the first time, describing the character also called "The Cigarette Smoking Man" or "CSM".

Actually Mulder uses the name "The Cancer Man" for the first time in the second season episode "One Breath".

Well, this might be more of a goof on X's part but, after X kills the two men near the end of the episode he puts the gun in one of the dead men's hands. This is wrong because when someone dies they won't hold on to anything they may have had in there hands they drop it. It would have been a better move to just drop the gun by the guy.

Agent Mulder mentions he is colorblind, as a reason why the encrypted code did not have an affect on him. In reality to become a Special Agent at the FBI he would have had to pass a color vision test.

Scully fires 6 rounds at the hotel room door, though we are later told she fired only 4.

There was no car next to the pole that Mulder climb before he climbed it. But when he comes down, he steps on a trunk of a car.