Revealing mistake: When evil Willow licks good Willow's neck the neck that you see is a lot longer than either of theirs. Since both Willows' faces cannot be filmed at the same time a body double must have been used for this shot.
Revealing mistake: After Faith brings Angelus back, when she goes to punch him and he blocks, the stuntman switch is very obvious - he's noticeably taller, has different hair and doesn't even seem to be wearing any vampire make-up.
Revealing mistake: In the scene where Buffy is fighting the big scary lunch lady, look closely in the far away shots because you can see that the big lunch lady is now a skinny little stunt double. (00:41:55)
Revealing mistake: In Graduation Day Part One, when Faith shoots Angel with the longbow, you can see the false point spring out as if it were on a hinge from inside his jacket. (00:22:47)
Revealing mistake: Towards the end, when Buffy stabs Faith, there is no blood whatsoever.
The Harsh Light Of Day - S4-E3
Revealing mistake: When Buffy is fighting Spike, she throws him against a table and then runs over it; you can see this is her body double. It is extremely obvious.
Revealing mistake: Towards the end when Oz says to Veruca, "You don't wanna find out what I am", and partially turns into a Werewolf. You can tell that both of the actors were filmed separately for the CGI's sake as Oz stands out from the picture more than Veruca does. The same effect is obvious during "Doppelgangland" in season 3 where there are 2 Willows onscreen at once.
Revealing mistake: In the scene when Oz stops the female wolf from attacking Willow, and they both change and Oz tears her neck out, if you look closely, you can see the actor's real skin at the neck.
Revealing mistake: Several times during Spike's escape from The Initiative, his reflection can be seen in the glass. Vampires aren't supposed to have reflections. (00:13:00)
Revealing mistake: One of the arrows that Spike gets hit with passes through just below his left shoulder, but there are times when you can tell that it is a fake arrow because the part of the arrow sticking out at the front moves and the part at the back does not. Look at the arrow just after Buffy turns Hus into a bear.
Revealing mistake: When the Gentlemen are chasing Tara in Stevenson Hall, in the first hallway two Gentlemen are floating down the corridor and you can tell they're on wires because the one on the screen's left "jumps" a little as if his wires got snagged.
Revealing mistake: In the scene where Giles and Ethan Rayne are drinking in the bar, the shots cut between Giles and Ethan frequently. When on Giles, you can see Ethan's hands/arms and his glass in the foreground of the shot, but almost every time the shot switches back to Ethan he's in a different position with no time to have moved. (00:14:40)
Revealing mistake: When Faith is being chased by Buffy she jumps over a brick wall but as she does the wall wobbles.
Revealing mistake: When Jonathan assists Buffy at the beginning of the episode, he enters through the glass ceiling of the mausoleum, which shatters into squares. Although the audio is the sound of glass shattering, the pieces of "glass" flutter down, revealing that the squares are paper - simply for the safety of the actor during the stunt. (00:05:00)
Revealing mistake: When Tara is being chased by the monster and she falls, she says a spell and white smoke comes out of her hands but it is coming out of the sleeves of her shirt, the way she holds her hands up suggests this as well.
Revealing mistake: After Buffy leaves the cave where Adam kills Forrest, she trips and falls down a horizontal slope. She then rolls over some rocks and one of the rocks wobbles.
Chosen answer: From what we see in the series, certain girls are identifiable as potential Slayers - Kendra clearly fell into this category. The identification method is presumably mystical in nature, but the Watchers' Council are pretty effective at that sort of thing, so they're quite good at tracking down the potentials ahead of time. Not perfect, though - it does appear that Buffy herself may have slipped through the net - certainly she had no inkling of what she was until she'd already taken on the role of Slayer. It is possible, however, that this was actually cultural - an American family would hardly be likely to turn over their daughter to some strange man for 'training', so the Watchers might have chosen to keep an eye on her covertly, whereas some other cultures (like Kendra's Jamaican parents) might be more willing to believe.
Tailkinker ★