Factual error: Mulder refers to a Royal Navy vessel that sailed from Leeds. No large ship can sail from there - it is at least 50 miles from the coast.
Graduation - S5-E24
Factual error: In the episode Class Pre-Union, they were the class of 2000. In the episode Graduation, they were the class of 1998.
Factual error: The originally-aired version spelled the word "Acquisition" without the "c" and it appeared in the credits as "Aquisition." This goof has been corrected in the DVD versions.
Factual error: Early in the show, 3 Marine officers are being questioned about the death of a 4th. All 3 are wearing the Marine insignia on their jacket lapels reversed. [The anchors should point IN, not out] That mistake is corrected for some characters later in the episode, however at least two of the officers are seen later without their jackets, and their shirts do not have military creases - 2 in the front, 3 in the back - standard for a Marine blouse.
Factual error: None of the Swedish wedding traditions Ingrid mention actually existed in the 19th century and the bread she's baked is not traditionally Swedish. When she later speaks in her mother tongue it's definitely not Swedish.