Plot hole: Bullets, which have been passing harmlessly through Nick for two whole seasons, suddenly do him great injury here. And though he's taken to surgery, the bullet lodged in his head is never removed. It's still there on the latest x-ray when LaCroix arrives to "consult" with the doctor.
Deliberate mistake: At the end, a line apparently cut earlier creates slight confusion. Nick says to Nat, "When LaCroix asked you why?" (why save vampires). In the earlier scene as shown, the elusive LaCroix spoke only briefly to Nat in the lab, and never asked her that question.
Plot hole: In a major violation of series canon, this episode ends with the vampire doggie (a silly enough premise to begin with) bringing its owner across, a process previously established as complex enough to be far beyond even the smartest canine's capabilities.
Continuity mistake: While the doctor is playing the tape for Reese, Nick and Tracy, the Toronto Leafs hockey team coffee cup on Reese's desk is sitting with the leaf logo turned away from the camera and the hockey player art showing. A few shots later, though Reese isn't drinking any coffee and hasn't touched it, the cup turns itself so the leaf logo is now visible. A little later, it's turned back again.
Factual error: At the end, Nick recites the "you have the right to remain silent" spiel to Rita as he arrests her. Suspects in Canada, where this show is set, are not "read their rights." That's a U.S. law (the Miranda Act) and it isn't observed north of the border.
Revealing mistake: When Robert is shot, the car blowing up in the alley is sort of half an explosion, abruptly curtailed. Severely cold weather during shooting literally froze the cameras and left only a few seconds of usable footage. That snippet is repeated for effect, but the explosion still looks "cut off" because it is.
Plot hole: Because he has no discernible vital signs, the wounded Nick is declared dead in the hospital. When he revives, he's rushed into surgery, where he'd surely have been reattached to a monitor. Somehow, though, no one on the medical staff notices that their patient still has no normal pulse or heartbeat.