Revealing mistake: When Gwendolyn Post fights Angel in the mansion, she breaks a long-handled shovel over her knee to create a stake. If you watch in slow motion, the handle of the shovel is pre-broken (she simply holds the two pieces together tightly), and the pieces separate just before they actually make contact with her knee. (00:34:55)
Revealing mistake: When Buffy saves the man from being hit by a car, there is a very quick shot of the man standing in front of the car. In this shot, Buffy can seen on the left hand side of the screen running into the street to save him. But the shot then immediately cuts to Buffy who is suddenly still standing on the other side of the street with Lily and runs into the street to save him again.
Revealing mistake: Near the end when the good Willow says goodbye to the evil Willow and hugs her, there is a shot of the back of the good Willow's head and the evil Willow's face. You can tell that the good Willow is a body double wearing a wig - there's something circular near her scalp, possibly an elastic band.
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.
Revealing mistake: Right at the beginning of the episode, Buffy is sat outside with Willow and Willow is levitating a pencil. The pencil is obviously CGI as it stands out from the rest of the picture and doesn't cast a shadow. It also disappears from view whenever there is a side-angle shot of Willow.
Revealing mistake: Both the good and evil Willows' faces are barely seen onscreen at the same time due to them being played by the same actress, but when they occasionally are the good Willow stands out from the picture more than the evil Willow. She must have been filmed separately from the good Willow and edited in later.