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.
Factual error: The two human hunters shown in "North Texas - 35,000 B.C." look to be Neanderthals. Neanderthals were strictly Old World. There may have been early Native Americans around Northern Texas then, though generally it is assumed that the Settlement of the Americas had not yet taken place or was in its very early stages.
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.