Visible crew/equipment: After Tara is found in the Janitor's closet and brought back to her room as Willow & Buffy come in, there is a mirror to the left of the door. It has a scarf draped over it to prevent it from reflecting things it shouldn't, but the bottom right corner is still exposed and in it you can see glimpses of the crew moving around behind the camera.
Continuity mistake: After the group find out Jonathon cast a spell, Willow is holding a book to her chest. But in the side shot, the book is resting on her stomach and then back on her chest a few shots later.
Revealing mistake: When Tara is being chased by the monster and she falls, she says a spell and white smoke comes out of her hands but it is coming out of the sleeves of her shirt, the way she holds her hands up suggests this as well.
Revealing mistake: When Jonathan assists Buffy at the beginning of the episode, he enters through the glass ceiling of the mausoleum, which shatters into squares. Although the audio is the sound of glass shattering, the pieces of "glass" flutter down, revealing that the squares are paper - simply for the safety of the actor during the stunt. (00:05:00)
Chosen answer: He had larangitis and because of the shooting specifications they had to have him shoot the scene with a bad throat, they then re-recorded his lines and dubbed them. At various points during the scene and indeed the episode you can see him touch his throat or rub it.