Continuity mistake: In the beginning of the movie, the doctor invites the two girls knocking on his door inside out of the rain because their car broke down. He gives them both water (laced with drugs). One of the girls knocks her glass over, spilling most of the water, which can be seen as the glass sits on the table. Then when the doctor leaves the room, while the two girls are talking, you can see that the glass on the table magically is filled again.
Continuity mistake: When Doctor Haier is explaining the operation to the captives, he says he'll sever the ligament in the knee. However, around the 1-hour mark when the captives are walking by the pool, you can see the rear woman's leg raise her left foot off the floor when she moves forward, showing she still has control over her knee.
Revealing mistake: When Dr. Heiter takes Katsuro from his car, you see Heiter drag him out of the back seat. He then proceeds to lift Katsuro. If you watch Katsuro's hand, you'll see he "helps" Heiter by pushing himself on the door-frame of the car (you see his hand tense as he pushes himself up).