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.
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.