Revealing mistake: Near the end of the episode when the Mayor releases the vampire from his cage and lets him cut his head in half, you can tell that the Mayor's head is a still image when it fixes itself. The expression on his face doesn't change at all and looks completely unrealistic.
Plot hole: Buffy saves Angel from Balthazar by dropping a light into the tub of water that he's sitting in. But why would the tub have been put directly underneath a light? Balthazar was being kept alive throughout the episode, wouldn't having a light above him be a bit unwise?