Other mistake: In the famous scene where Buffy tells Giles, "If the Apocalypse comes, beep me," she grabs her pager from out of shot. Trouble is, she's standing in the middle of her hall and there's nowhere she could have picked the gadget up from.
Other mistake: The Master's skeleton, before it is destroyed, looks to be female and not male. (Note the wide hips and large birth canal-like hole.) Also in the Next season's "When she was bad," they have another shot of the same skeleton, however the hips are much smaller and more male.