Factual error: Nick tells fellow vampire Aristotle, "You still owe me for that time at the Battle of Hastings." Nick was brought across in 1228, or so the intro told us every week. The Battle of Hastings, as any British school kid knows, was fought in 1066, over 100 years before Nick's mortal birth.
Factual error: Nat's god-daughter Cynthia Luce is buried within a few days of her murder with an elaborate headstone already in place. (In reality, headstones take months to produce.) But the wrong name is carved on the stone. It reads "In memory of Cynthia Lambert."
Factual error: At the very end, Bridget Hellman is buried with a fully-engraved headstone in one just one day's time (which happens only in tv & movies: the process takes months in real life... er, death). She also spontaneously returns as a vampire without being "brought across," a violation of series canon, and she comes back mysteriously sporting 3 times more hair than she had as a mortal.
Factual error: The cop who arrests Nick reads him his Miranda rights ("You have the right to remain silent," etc.) But "Forever Knight" is set in Toronto. At last report, Toronto rather resolutely insists upon remaining in the sovereign nation of Canada, where the Miranda Act, a U.S. Federal law, is not observed.