Continuity mistake: When Scully walks into Mulder's office, she stops in the position with the "I want to believe" poster in the background, then she takes a couple of steps to make acquaintance with Mulder, and she finds herself again in the position with the poster in the background.
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.