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. (00:39:50 - 00:41:32)
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: At The Beginning of the episode we see Duane being abducted and he begins to scream. Look at the "alien" on the left of the screen near to the top of the bed and you will see him move his head revealing where the suit ends and neck/head begins. (00:02:12)
Revealing mistake: At The Beginning of the episode Darlene burns her hand trying to open the door, she then puts on an oven mitt and opens the door. If you look closely though she still uses her other naked hand as well as the covered hand to open the door and her naked hand is completely unhurt.
Revealing mistake: When Agent Reyes drives up to Agent Dogget's parked car, he looks in his rearview mirror. Its background should be obviously a transparent windshield to which the mirror is fixed. However, when the camera shows the image seen in the rearview mirror, we see in the background something similar in appearance to a solid wall. (00:10:50)