Nightmares - S1-E10
Revealing mistake: The Master's artificial thumbnail is visibly falling off in a few shots of the opening teaser. (00:01:30)
Nightmares - S1-E10
Revealing mistake: When Xander punches out the evil clown if you freeze-frame you can see the punch missed the clown by quite a distance, I noticed it without freezing it but it's better to see if you do. (00:35:35)
Nightmares - S1-E10
Revealing mistake: When Willow opens the door to the basement to look for Buffy, the shot looking up the basement's staircase shows Willow's shadow on the inside of the door. In the previous shot when she opened the door, it opens towards a wall and a staircase to the second floor. But there is no light source coming from the wall or the staircase, giving the impression that the basement was filmed on a separate set.
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.