Factual error: The scenery is set in Berlin, following the 1989 Re-Union of Germany. Only thing is: the guards at the archive building use AK-47s, while after the Re-Union, the East German National People's Army and the West German Bundeswehr (Federal Defense Force) were joined - and the West Germans don't use any AKs.
Continuity mistake: When Sydney has the "Book of Love" from the limo she starts to read it. In one shot she holds the book firmly with her hands wrapped around it. But when the motorcyclist grabs it she is holding the bottom of the book in the tip of her fingers [To aid the stunt].
Other mistake: When the bad guy at the start of the episode runs to chase Sydney and Nigel, his foot kicks the side of the large stone lid which cause it to lift up. If this large piece of stone was real stone it would not have moved at all from being nudged by a foot.
Factual error: Entering the secret room in the old part of the castle, the protagonists find there the ring and a specific painting...but doing so, they casually walk in front of another painting kinda hidden in the background...a perfect copy of the uber-famous Leonardo Da Vinci painting "Lady with an Ermine"! Considering that the painting was finished in 1490, the opening caption of the episode said "1430." Even if after the incident the young lord lived into his 80s or 90s, there surely wouldn't be a perfect copy of it in his secret hideout. (00:25:40)
Other mistake: In the Romeo & Juliet-like beginning of the story, Calum looks at his own hand and discovers Elena's blood. Obviously censorship ratings prevent the scene from being realistic and he has barely a smudge of blood on his own hand, but Elena was lying dead on top of him; the fact that his clothes are absolutely pristine for the rest of the scene is laughable - at least she could have gutted herself a little bit on the side and not where she would inevitably bleed him a river right on his white shirt. (00:02:55)
Plot hole: Lori managed to get to the bunker through the elevator because the modern board was still replaced by the antique one. But that implies that she was able to call the elevator as normal. And if people can just call the elevator back but the normal control board has been replaced, then they wouldn't be able to use the elevator as normal, creating quite a problem in a huge hotel. Sydney earlier was not able to switch the board back to the normal one since she went out from a (mysterious?) exit/entrance, so this situation makes no sense.
Other mistake: The episode begins with a caption; "Chicago, 1930." In present day, through internet magic (hotel records from early 1900s somehow digitalized and publicly available!) Nigel and Sydney talk about the history (fictional, no actual historical basis) of the Roxbury. Nigel says that the original owner went out of business "after the war", then the company was sold, and the hotel refurbished, despite being, Sydney says, "only 5 years old." Later in the episode, Sydney looks at a plaque from the Chicago Historical Board that dates the elevator as "circa 1901." (00:12:35)