Factual error: Mallory tells Florence that the school is in Cumbria. Cumbria didn't exist in 1921. It was created in 1974 from Cumberland, Westmorland and part of Lancashire.
Continuity mistake: In the scene where Lily is hiding behind Thomas' computer desk, she uses her little Eiffel tower necklace to put inside the socket to short out the power. When she goes to put it in, you can see the computer is plugged in on the bottom socket. After a really quick cut scene, the computer is now plugged in on the top socket. (01:05:00)
Continuity mistake: After Sarah finds her father in the bedroom and goes downstairs to get help, the blood pattern on her shirt/upper breasts changes. While the movie is edited to look like one continuous shot, it was filmed in bits and pieces, and the blood pattern was not consistent in some of the shots, it seems.
Factual error: Colonel Meyer's promotions are listed in his SS ID card as Hauptmann, Oberstleutnant and Oberst. Since the SS used a different ranking system from the Wehrmacht, they should actually be Hauptsturmführer, Obersturmbannführer and Standartenführer.
Revealing mistake: As the guy is being pinned down by the giant hooks you can see for over a minute he can simply escape and slip his hands out. (00:52:30 - 00:53:30)
Factual error: In his crude attempt to replicate the Human Centipede surgical procedure, Martin employs several handyman tools, including a carpenter's staple gun (using T50 staples as sutures). Problem is, carpenter's staples would never join soft human body tissues, they'd pull right out with little effort (T50 staples only join to very solid base materials, such as wood). Martin's victims, hysterical as they were, could have freed themselves from the T50 staples almost immediately.