Revealing mistake: When Art is hitting the Trevor (the blinded man) character with the hammer/hatchet in the beginning, if you look closely, you can see blood coming from Trevor's head, yet there is no wound or cut. The blood is actually coming from the prop that is hitting him in the head. (00:09:00)
Revealing mistake: When Cecil's face is being cut by the Knife-chair, the first time he comes into the blades, it is painfully obvious that the blood is being pumped through the knives, and that his face is not being cut. You can even see his bloody, yet cut-free, face almost come through the knives for at least two full seconds, before there is a flash-frame jump-cut, and a number of deep cuts magically appear on his face. (01:12:50)
Revealing mistake: When Rigg shoots Art, just before that you can see Art shouting. Look at his chest. The bullet wound is already there (no slow-mo needed). (01:24:40)
Revealing mistake: When Ivan impales his right eye, you can tell that the eye is fake because he doesn't so much as flinch when the blade is coming down. (00:45:00)
Answer: It plays an important role in the next few movies.