The Harsh Light Of Day - S4-E3
Revealing mistake: When Buffy is fighting Spike, she throws him against a table and then runs over it; you can see this is her body double. It is extremely obvious.
Revealing mistake: Towards the end when Oz says to Veruca, "You don't wanna find out what I am", and partially turns into a Werewolf. You can tell that both of the actors were filmed separately for the CGI's sake as Oz stands out from the picture more than Veruca does. The same effect is obvious during "Doppelgangland" in season 3 where there are 2 Willows onscreen at once.
Revealing mistake: In the scene when Oz stops the female wolf from attacking Willow, and they both change and Oz tears her neck out, if you look closely, you can see the actor's real skin at the neck.
Revealing mistake: Several times during Spike's escape from The Initiative, his reflection can be seen in the glass. Vampires aren't supposed to have reflections. (00:13:00)
Revealing mistake: One of the arrows that Spike gets hit with passes through just below his left shoulder, but there are times when you can tell that it is a fake arrow because the part of the arrow sticking out at the front moves and the part at the back does not. Look at the arrow just after Buffy turns Hus into a bear.
Revealing mistake: When the Gentlemen are chasing Tara in Stevenson Hall, in the first hallway two Gentlemen are floating down the corridor and you can tell they're on wires because the one on the screen's left "jumps" a little as if his wires got snagged.
Revealing mistake: In the scene where Giles and Ethan Rayne are drinking in the bar, the shots cut between Giles and Ethan frequently. When on Giles, you can see Ethan's hands/arms and his glass in the foreground of the shot, but almost every time the shot switches back to Ethan he's in a different position with no time to have moved. (00:14:40)
Revealing mistake: When Faith is being chased by Buffy she jumps over a brick wall but as she does the wall wobbles.
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)
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: After Buffy leaves the cave where Adam kills Forrest, she trips and falls down a horizontal slope. She then rolls over some rocks and one of the rocks wobbles.
Revealing mistake: In Xander's dream, when he climbs into the back of the ice cream van, for a split second before the window goes out of view the backdrop cuts out and the blue screen is visible. Only visible in widescreen version.
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.