Factual error: LaCroix visits the comatose Nick in the hospital and detaches a pulse monitor that A) shouldn't be beeping, seeing as it's attached to a vampire, and B) should have immediately sounded an alarm when it was removed. (00:08:30)
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)
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)
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)
Visible crew/equipment: While Natalie is stocking Nick's kitchen with groceries, a shiny pot on the shelf over the stove captures a reflection of several crew members standing on the other side of the set. (00:24:55)
Visible crew/equipment: When Natalie is telling the amnesia-stricken Nick that he's a vampire, an equipment shadow moves across the stove in the lower right hand corner of the screen. (00:35:15)
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.