Visible crew/equipment: Season 6, Episode 'Villains,' when Willow is chasing Warren through the forest, trees move out of her way to let her pass. When the trees behind her move back into place, in one shot you can see the rope they used to tug one of the trees down.
Revealing mistake: As Willow walks through the forest pursuing Warren, just after we first see Warren running, there's a shot of Willow walking into a bit of a clearing. Her shadow crosses a tree on the right (could be caused by the moon), but then a couple of seconds later another shadow is visible on a tree to the left - obviously an artificially lit forest. (00:33:15)
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.