Continuity mistake: In the spaceship, Mulder unbuttons Scully's jacket to perform CPR on her. The coat is undone about 13 cm under her chin. Later, it is clear that the coat is buttoned up again, up to under her chin.
Continuity mistake: When Mulder and Scully start frantically running from the bees inside the dome Scully is clearly seen untying her belt twice.
Revealing mistake: In the end when Mulder is reading the paper in the park. The paper is clearly a prop. Only the frontpage is real - when he puts the paper away you can see that the rest of the paper consists of blank pages.
Continuity mistake: As Mulder and Scully enter into the building that houses the bees is seems Mulder's tie gets blown over his right shoulder and then he is seen inside and it is over his left shoulder.
Factual error: Dallas, Texas is not surrounded by desert mountains. This is a dumb tourist mistake; when that shot of downtown Dallas appeared on the screen audiences there groaned in unison.
Visible crew/equipment: Near the beginning when the "cavemen" are looking around after hearing the noise from the alien, you can clearly see what looks like a sneaker print in the ice, in 35,000 BC.
Factual error: North Texas was never covered by glaciers. The glaciers never got south of Kansas. And there weren't any mountains in Dallas then, either.
Continuity mistake: At the end, Scully is talking with the OPR people. Her suit is dark blue like in the first meeting. The next shot she goes out to meet Mulder. She is wearing another suit. That one is a different cut and is a very dark green.
Factual error: Mulder and Scully sneak into the morgue at Bethesda Naval Hospital to view the body of the dead fireman from the bomb blast. Mulder lifts the sheet back on the body to show that the man's tissue is "like jelly." Scully surmises there's been some kind of cellular breakdown, and then says the tissue is "completely edematous." However, Dr. Scully mispronounces "edematous" as ed-a-may-shuss. The correct pronunciation is i-dem-a-tuss.
Continuity mistake: In Mulder's apartment hallway, when he and Scully are about to kiss, she let out one tear from her right eye. The next shot we see Scully, the trail of the tear has disappeared and also has the tear.
Continuity mistake: When Stevie the little boy is in the cave, there are basically two shots: One from behind his three friends looking down, and one from Stevie's perspective, looking up. Every time we look down into the cave there is quite a lot of sand trickling down, but when we are with Stevie in the cave looking up, there's no sign of it.
Revealing mistake: When the booby-trapped car explodes, the detonating wire is visible running out of the exploding car and out of shot.
Factual error: The savages shown in "North Texas - 20,000 B.C." look to be Neanderthals. Neanderthals were strictly Old World, although there may have been Native Americans around N. Texas then.
Factual error: The stamp of the scene says that it is Wilkes Land, Antarctica. Wilkes Land extends from Cape Hordern in 100°31' E to Pourquoi Pas Point in 136°11' E. However, the coordinates of the spaceship are 63°00' E, 83°00' S, so this place is located definitely outside the area of Wilkes Land.
Continuity mistake: When Mulder and Scully stop by the fence after chasing the tanker trucks in Texas, Mulder takes out a map, opens it then folds it and looks at it closely. The map looks folded differently in the two subsequent shots, then is shown wide open again in the third shot when Mulder starts folding it again.
Continuity mistake: In the Dallas lab, Mulder takes the test-tube that contains the fossils and give it to Scully. Then, he gives it to her again.