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.
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.