Revealing mistake: When Kyle goes into the dead-body room, there is a shadow of a hanging dead person on the wall next to him. The shadow of the open door on the wall on the other side of him shows that the shadow of the body shouldn't have been there as the light would have to have been coming from the very far wall. When Kyle is looking around you can see that the only light source is the high windows on the left side of the room.
Revealing mistake: When Kirsty is confronted by the Cenobite doctor, Dr. Channard, there is a huge gust blowing towards her. If you look behind her, there is a suppossed stone wall, which appears to be flapping in the wind, revealing it is merely a fake set wall.
Continuity mistake: When Julia emerges from the mattress and kills Browning, the mattress becomes completely saturated with blood. Then when the mover comes to take it away at the very end, there are patches where the mattress is clean. Then again, when he is pulled into it and his friend comes into the room, it is saturated again.
Revealing mistake: When Tiffany almost falls off of the ledge, the ledge wobbles violently when she grabs it.
Visible crew/equipment: When Tiffany almost falls off of the ledge, you can briefly see a wire on her back.
Continuity mistake: When Tiffany goes back to retrieve the diamond shaped box from Julia's grasp, the position of Julia's fingers around the box change between shots, as Tiffany reaches down to grab it out of her hand.
Answer: All we can see (in abstract closeup) is serpent-like tendrils cutting him and driving the pins into his head. As someone else said, it's likely another cenobite, although alternately it could also be the same "contraption" that turns Channard into a cenobite late in the film, given the tendrils are similar to the ones he sprouts.