Continuity mistake: In the flashback to 1228, Nick's just-awakened sister Fleur has her hair down over her ears, and is wearing a white night dress that covers her to the neck. Cut to a close-up and her hair is pulled back to reveal pierced earrings (unlikely in 1228, and did she sleep with them on?), and she's suddenly in a bare-shouldered gown instead of her night dress. (00:08:35)
Continuity mistake: Nick and Schanke are interrogating a murder suspect and disagreeing about whether or not she did the deed. They leave the interview room together, and Nick's hair completely changes styles as he walks out of the room. On the inside of the door, it's combed down over his forehead and rather "fluffy." The minute he exits on the other side of the door, it's brushed back and slicked down. (00:25:55)
A More Permanent Hell - S2-E20
Revealing mistake: Mt. Vesuvius is erupting and LaCroix's villa is collapsing around him. But while the ground "shakes," statuary falls over and the walls cave in, look closely at the fountain/pool in the center of the room. There's not a drop splashing out of it. In fact, the water's surface is smooth as glass.
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)
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)
Factual error: In the flashbacks, the shipboard lifesaver rings are emblazoned with the incorrect designation "S.S. Titanic." The Titanic was a Royal Mail Ship. The stencil should have read R.M.S., not S.S. (00:14:00 - 00:24:50)
Plot hole: The insertion of a musical sequence used to fill extra time in this episode creates a plot problem. It's spliced in between Nick's urgent plea for Nat to stall the autopsy and his rescuing the bound and gagged Rebecca, and unfortunately makes it appear that in that interval, Nick simply went home, played music and sat in his loft window brooding, ignoring the case altogether. (00:40:40)
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)
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: 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)
Continuity mistake: The actor playing the bad guy apparently didn't stick to the script. What he says on the phone, telling Tracy where to meet him, and what she writes down for Nick to find later, don't match at all. (00:35:00)
Revealing mistake: When Nat enters Nick's loft from the lift, the bright lighting reveals that there is no break in the solid floor to allow the movement of a real elevator. (00:36:45)
Blood Money - S2-E17
Continuity mistake: When Sean sneaks back into the night club owner's office, we can see as he enters that there is no one in the left-hand side of the room. Yet, as Sean sits down at the desk, Walken somehow emerges from the left corner, which we have just seen was empty a moment before. (00:39:15)
Continuity mistake: The blood stain on Tracy's lapel changes its shape several times between takes. (00:08:15)
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)
Blood Money - S2-E17
Continuity mistake: When Nick and Schanke begin interrogating Sean, the tape recorder on the desk isn't running. It's going a few shots later, though, even though no one's touched it to turn it on. (00:30:30)
Plot hole: At the Precinct after the second spa incident, Captain Stonetree refers to "two unexplained reflex murders in one night." But the first victim survived the attack - so there was only one murder, not two. A little worse than a simple character mistake, since he's the police captain and knows the case. Schanke gets it right later, when he says, "One murder and one attempted murder." (00:14:20)
Father's Day - S2-E7
Continuity mistake: During his fight with Constantine's henchmen, the bullet holes in the back of Nick's vest disappear and reappear several times between shots. (00:23:30)
Plot hole: In the morgue, Nick tells Nat that vampires disappear when they die. Not so in this series. "Forever Knight's" vampires, defying old movie clichés, remained stubbornly intact when staked, making Nick's statement here a glaring contradiction of canon. (00:16:05)
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)
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.