Revealing mistake: In the scene where Kirstie is in Hell searching for her father, she is running up and down Hell's corridor's, becomes winded and stops. As she does so, she slaps herself against a concrete corridor wall; when she does this, you can see that the "concrete" wall looks more like a rubber covering, as it moves with her. (01:01:50)
Revealing mistake: When Julia takes the cigarette from the doctor in the room with the mirror, you can see, if you look closely, that there is no blood on the cigarette. But, the hand she has the cigarette in left some blood on the hand of the doctor. (00:36:10)
Continuity mistake: When Browning lies down while cutting himself, the mattress under him is dry and his blood is only smeared over his chest. Yet when the shot cuts to Channard and back, the mattress has suddenly become utterly soaked in Browning's blood. (00:30:00)
Visible crew/equipment: When Tiffany almost falls off of the ledge, you can briefly see a wire on her back.
Continuity mistake: When Dr. Channard is being turned into a Cenobite, his tie disappears in some shots. (01:04:45)
Revealing mistake: When Julia first emerges from the mattress and is scrabbling on the floor, you can see the body double is actually wearing a thong. (00:30:35)
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.