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)
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)
Other mistake: Nick pronounces the plastic surgeon's name as "Yergen" in one scene and as "Jergen" in the next. At the end, she becomes Dr. "Yergen" again. (00:29:40 - 00:31:10)
Factual error: Nick's '62 Cadillac had an attached police radio, but never had a car phone, nor the giant external antenna attachment they required in the early 90s. Yet Schanke somehow makes a call from Nick's "car phone" in this episode.
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.