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: Presumably another cenobite/cenobites. The choice not to show them makes for a better scene, as that moment is all about him, becoming the iconic Pinhead; the cenobites who made him that way are of no consequence to the story, and their own grotesque appearance would have distracted from his transformation.