Factual error: At the crime scene where Roland killed one of the professors by dunking his head in liquid nitrogen and dropped him on the floor, someone has drawn an outline of the dead prof's body and all the bits his head broke into when Roland dropped him. This outline-drawing doesn't happen in life because it interferes with the crime scene, it's a popular myth.
Factual error: When Mulder gives Scully the birthday card, she says that he is two months early. Given that Scully's birthday is on 23 February, the time of the episode should be sometime near Christmas. However, through the whole episode, there is no sign of Christmas time (not a single Christmas tree, no Christmas decorations, etc). (00:19:35)Chop Luftmysza
Factual error: According to the NASA space shuttle launch countdown procedure, the close-out crew retreats to fallback area in the "T-3 hours and counting" phase. The members of the crew are the last persons who leave the launch pad at least 20 minutes before liftoff. However, during the close-up take of the main engines, when the ground control says "All systems go for APU start", we can see two members of the ground personnel in orange suits near the engines. (00:12:40)Chop Luftmysza