Revealing mistake: While the computer is being infected with the virus, Peterson turns away from the screen to face Scully. Two seconds later, after the shot of Scully, the same shot is shown again but in reverse so that Peterson turns back to the screen. It's more obvious if you look at the weird way his eyes blink in the reversed shot.
Revealing mistake: When the doctor examine Ronnie's "corpse" in the morgue, he pulls out the stake. It is quite obvious that he's struggling with a wooden stake that is placed on the actor's side, not through his chest. Moreover, the camera pans up and you can briefly see Ronnie's unharmed chest.
Revealing mistake: The answering machine at the end appears to not be plugged into any phone line. The shot shows the phone jacks on the right side of the machine are empty. In order for an answering machine to work at least one of those ports would need to be hooked to the phone.
Revealing mistake: It seems that in the first shot the doctor injects something into the vaccine vial rather than extracting from it since the syringe plunger is moving inwards instead of outwards. In the next shot one can see that the syringe is being filled with the vaccine, however the arrangement of doctor's fingers is suitable for injection, which strongly suggests that the scene is played backwards.
Revealing mistake: The establishing shot of Boston General Hospital at the end of the episode shows the same building that was used for the BYU Medical Center in Provo, Utah eight episodes earlier in "Roadrunners" (note the vegetation under the part of the building that extends over the curb and the lettering on the building that says "Entry A"). The actual building is presumably in Kansas since the Kansas state flag was visible outside the building in the aforementioned episode.