Other mistake: The surname on the envelope is "Szczesny" (which is of Polish origin and means "Lucky"), so its pronunciation should be something like "shtcheznhy." However, in the subtitles the surname is written as "Szezesny" and pronounced by Scully accordingly.
Continuity mistake: Later in the episode a man bangs on an air conditioner to try and stop it from making noise. In the first scene you can tell air is moving because the streamers are moving. After he bangs on the unit the streamers stop moving. The air conditioner is still on.
Factual error: When Mulder wants to shoot the third resurrected agent, it turns out that there is no ammunition left in the revolver. In fact, there should be one bullet left. When Frank Black takes the handgun from the necromancer, one can see that the cylinder is full with six bullet. Then, he fires three bullets into the first resurrected agent and two bullets are fired by Mulder into the second resurrected agent.
Factual error: When Mulder is visited by his mother, he definitely shows brain activity, because he is "talking" with her, however there are only flat lines on the monitor. In a similar situation in "The Sixth Extinction" episode, when Kritschgau and Skinner ask him questions and he anticipates them, the monitor shows intensive bursts.
Continuity mistake: When Skinner walks away from the conference table, his armchair is only slightly turned left from its normal position towards the table, but when he comes back with additional documents, the armchair is turned left to the position perpendicular to the table.
Other mistake: Padget hands over one page of his book to his imagined character who is invisible to anyone else except him. Therefore, the page should hover in the air for an external observer. However, Scully watches his apartment on a monitor and notices nothing strange.
Visible crew/equipment: When Mulder and Scully are talking to the owner of the trailer park and he is berating Mulder, you can see the edge of the lens in the upper left corner of one shot. This is because the lens used was of a wide angle almost fish-eye type and the edge of the lens itself can occasionally creep into the frame on most cameras.