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: 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?
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. (00:00:05)
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: When LaCroix arrives at the amnesiac Nick's loft, Nick recognizes him as "the doctor from the hospital." But Nick was still comatose when LaCroix posed as a doctor and came to his room, so he shouldn't remember that particular "doctor." (00:19:45)
Blind Faith - S3-E5
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. (00:40:00)
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)
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. (00:05:05)
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.