Factual error: The murderer works for a blood bank and drives a bloodmobile, a plot holdover from the first pilot ("Nick Knight"), which was set in Los Angeles. This second pilot, however, is set in Toronto - and Canada doesn't have mobile blood units. (00:31:35)
Factual error: The murder victims, all Catholic, are specifically said to have "all worn crucifixes." But the jewelry Nat hands Nick as evidence, the cross that burns his hand and the necklace Magda later gives him are all plain crosses, not crucifixes. (The former is unadorned; the latter has a Christ figure on it.)
Factual error: The doll that supposedly belonged to Erica 300 years ago is too obviously modern, with a plastic head and body and nylon hair.
Spin Doctor - S1-E15
Factual error: Nick tells Nat about his past life as an assistant professor in the U.S., and refers to himself as "a certain indigenous vampire." Nick wasn't indigenous (native) to either the US or Canada, where the show was set. He came from Brabant, now part of Belgium. They were probably trying for "itinerant" (wandering), which would accurately describe Nick's long life.
Factual error: Vampire Erica commits suicide by sitting on a park bench at sunrise. Nick and Schanke are called to the scene only a short time later, or so we're led to believe, yet it's still fully dark when they arrive. No sunrise in sight. (00:01:00 - 00:05:30)
Factual error: In the flashback, when Nick regenerates on the autopsy table, his shredded clothes repair themselves, too. Maybe Nick buys his outfits in the same place the Incredible Hulk does.
Factual error: The supposed ballerina in this episode's flashback to 19th century France is not employing any real ballet steps or arm movements. All she's doing on stage is standing still and waving her arms in the air, performing something that looks rather more like semaphore than ballet. (00:10:10)
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.