Continuity mistake: When Cal falls against the canister in the lab and accidentally releases the poison gas, the position of the big yellow lever on the gas pipe shifts back and forth by 90 degrees between takes. (00:34:25)
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. (00:42:20)
Factual error: The costumes in the flashback to Nick's wedding are a mishmash of the mismatched, coming from several different eras, including a full metal suit of armor that's at least 2 centuries ahead of the flashback's period setting in the 1200s. (00:14:00)
Factual error: In the flashback to Nick's wedding night, his medieval tunic/shirt has 20th Century white plastic 4-hole buttons sewn to the cuffs. (00:29:25)
Plot hole: Nick has spent two seasons with the supernatural ability to see perfectly well in the dark. But here, exploring the haunted house alone, he suddenly needs a big neon-blue-beamed flashlight to see where he's going. (00:31:30)
Plot hole: Nick mysteriously forgets that he can fly in this episode. With the ghosts of his past victims in hot pursuit, he trips and tumbles all the way down a full flight of stairs. Any other time, he'd simply have flown away. (00:38:10)
Plot hole: The handyman tells the police that he has turned on the power to restore lights in the house. Why, then, is everyone still crawling through the place with flashlights?
The Games Vampires Play - S3-E15
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. (00:41:45)
Continuity mistake: Twice in this episode, Janette refers to having been a vampire for 800 years. Previous episodes established that while Nick was 800 years old, Janette was 1000. (00:14:45 - 00:33:00)
Plot hole: Throughout the first two seasons, Nick was always able to sense other vampires and to instantly differentiate between vampires and humans. Here, he inexplicably fails to detect that the returned Janette is now mortal. She has to tell him. (00:27:10)
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. (00:32:00)
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. (00:22:30)
Revealing mistake: When LaCroix opens the box containing Divia's little "surprise," the severed head's eyes move slightly as the covering tissue paper is removed. (00:07:20)
Answer: Nick was sick and tired of being an immortal bloodsucker. He wanted to be human, fall in love, get married, have children, grow old and die. As for Janette, according to her, she fell in love and the passion she felt "cured" her of her blood lust.