Revealing mistake: During the song-and-dance number "They Got The Mustard Out", if you look closely at the dancers, the man in green to camera right of the second row is horribly out of step with the music, and with the rest of the group. The spell makes everyone dance far better than they naturally would (Tara loses the clumsiness she often displays, for example), so it is a mistake to have someone under the spell dancing so badly.
Revealing mistake: The knives Giles throws during Buffy's training are a very poor CGI job if you watch it in slow mo. All three knives Giles takes off the wall are identical, with a broad curved blade and a plain black handle. The first one has a transparent blade as it flies toward Buffy, then we hear it bounce off the far wall and it's way out of scale for the distance. The second knife has a diamond-patterned grip instead of a plain one. The third knife conveniently spins in midair for a second so Buffy can kick it out of the way.