Visible crew/equipment: When the wolf begins to transform back to evil, as the wolf tries to take the wooden stake out of his heart, he leans forward and you can actually see the human arm behind him. He can't have an animals arm and a human arm at the same time - the arm is of an operator working the animatronic.
Continuity mistake: When Charlie and Amy run into the nightclub to get away from Jerry she is wearing a blue purple plaid jacket to match her skirt and blouse. The jacket disappears when Jerry takes her to the dance floor, and she is not carrying it or wearing it when Jerry makes his escape from the nightclub, kidnapping Amy. Then in the scene when Amy wakes up in Jerry's house dressed in the white gown, she begins to focus and look at the portraits on the wall. Directly below and to the right of the first portrait are her clothes, folded neatly over the back of a chair - purple skirt, blue blouse and the matching plaid jacket, which has magically reappeared.
Revealing mistake: Near the end, when Peter stakes Jerry while he is lying in his coffin, it's visible that the stake is held under his armpit and not into his chest.
Revealing mistake: Look carefully when Chris Sarandon jumps through the stained glass window - you can see it is his stunt double and not him, plus the stunt double is wearing gray sneakers instead of gray dress shoes.
Continuity mistake: When Peter presses the cross to Evil's forehead, it's level. But, in the shot when he pulls it off, it's tilted to create the X burn.
Other mistake: When Amy and Evil are at Peter Vincent's house to convince him to help Charlie, you see them looking into a horizontal mirror that's on the wall on the left side of the screen. If you actually look at the reflection, Evil is on Amy's right. However, he's on her left in reality.
Continuity mistake: When Charlie and Peter find the bitten wounded Amy lying unconscious on the floor in Jerry's bedroom, she is shaking as she transforms into a vampire. Her hair also changes back and forth between the shots. From a perfect curly coif to a longer frizzy vampish do. Then when Peter enters the room alone, and she gets up, it's suddenly long and curly down her shoulders.
Chosen answer: I would assume a "thrall" basically a ghoul that is attached to the vampire. A ghoul in old lore was created when a vampire gave blood to a living person, but did not take their blood first. They were then their daytime slaves, doing errands, finding victims for them, and took care of the vampire's body when the sun was out.
Billy was to Jerry... what Renfield was to Dracula. Slave, daytime watcher.