Factual error: In the opening sequence, we see a nuclear attack submarine beneath the ice in the Arctic Ocean. When the captain orders the ship to surface, one of the crewmen mutters, "Surface to what? We're under 32 feet of glacial ice!" Except that they're not under "glacial ice," they're under Arctic sea ice, which ranges from about 6 feet thick in the summer to about 15 feet thick in the winter.
Roadrunners - S8-E4
Factual error: At the episode's end, it shows Scully recuperating at the BYU Medical Center in Provo, Utah. However, on the flagpole outside of the center, you can see the Kansas state flag. (00:42:20)
Factual error: Gonzales shot himself in the forehead but he's got the back of his head intact - no exit wound. (00:27:40)
Factual error: In the background the full moon is visible and it's early morning (approx. 6:00 AM). However, at the start of the episode we see the full moon at night (approx. 19:50). Due to the fact that these are consecutive days, this situation is impossible from an astronomic point of view. (00:14:35)
Factual error: The last number on the truck's license plate is "77." On Russian plates the last two/three digits stand for a registration place. "77" is a number for Moscow, but the Gulag is located in Krasnoyarsk region, so the number should be different. Besides, the format of the plate (LDDDDL|DD) is incorrect - the correct one is LDDDLL|DD (L - letter, D - digit). (00:11:05)
Tunguska (1) - S4-E8
Factual error: This is probably the first time in history of civilized mankind that a person with a diplomatic visa is full-body-cavity searched, not to mention the illegal opening of the diplomatic bag. (00:04:05)
Factual error: Mulder tells Scully that he is trying 27828 and it is not working. It is not surprising that it is not working, because the Napier constant is not 27828, it is 271828. Plus they somehow they know how many digits from this constant should be entered on the keyboards. (00:15:55)
Soft Light - S2-E23
Factual error: Dr Banton's shadow devours the second policeman, however Banton cannot cast a shadow in that direction. (00:11:35)
Die Hand die Verletzt - S2-E14
Factual error: The loss of blood Shannon suffered was definitely insufficient to be the cause of her death. (00:25:00)
Factual error: In the opening scene it says it takes place in West Virginia, as the Army unit executes the hybrids, you can see their unit patch on their left shoulder. The patch is a diamond shape with a sunburst in the middle, this is the patch of the 40th Infantry Division of the California Army National Guard.
Factual error: The episode is supposed to take place in Traverse City, Michigan, United States. Mountains are visible in the sequence when Mulder is chasing after the killer on foot. The show was shot in Vancouver, British Columbia.
Factual error: When Byers asks Langly to try under active cases to find Holly Modeski in the FBI database, Langly only presses two or three keys on a keyboard, which is obviously not enough to enter "Holly Modesky" to the search engine. (00:19:40)
Factual error: When two unidentified men who approached Susanne Modeski in the warehouse take out their handguns, they have regular pistols. However, when they start to shoot, continuous fire is heard that could only be produced by an automatic weapon. (00:28:40)
Suggested correction: Fully automatic pistols exist, such as the Glock 18.
Nothing Important Happened Today II (2) - S9-E2
Factual error: Deputy Director Alvin Kersh tells Agent Doggett a story about King George III, who, on July 4th, 1776, the same day the American colonies declared their independence, allegedly made an entry in his diary: "Nothing important happened today." However, according to Arnold Hunt, curator at the British Library, King George III never kept a diary. (00:37:15)
Factual error: When Mulder is walking out of the cabin in Spender's uniform, the soldiers try to stop him. This is impossible since he is an officer (a lieutenant judging from the shoulder-straps) and they are privates, so they cannot give him orders. The only thing they can do in this situation is to salute him. (00:08:40)
Musings of a Cigarette-Smoking Man - S4-E7
Factual error: In the US classified information system there is "compartmented information", but there's no "compartmentalised information." (00:08:00)
The Sixth Extinction (2) - S7-E1
Factual error: When Skinner is at Kritschgau's apartment building, the time stamp onscreen says it's 5:05 AM, but when Kritschgau answers his door, he says it's 6:00 in the morning. Plus it is shown to be broad daylight outside. The sun is never up that early in Washington, D.C. (00:15:55)
Factual error: Skinner says that he could not print the file. However, Scully somehow must have been able to print it in order to give a hard copy to Albert Holsteen for his translation. (00:26:35)
Anasazi (1) - S2-E25
Factual error: It is completely impossible the Thinker has made a digital copy that cannot be duplicated.
Never Again - S4-E13
Factual error: The first time we see Ed's tattoo, it is supposed to have been freshly done and he even removes the new bandage. However, there is no blood or excess ink on his arm, no redness around the lines, no swelling at all; no way at all that it could be a fresh tattoo.