Revealing mistake: When Spike is playing with the Buffybot in his crypt and has her throw him onto his bed to stake him, the shot of him landing on the bed and the Buffybot climbing on top him is portrayed by stunt doubles. The shot is then interwoven with another shot of the actual actors. But it's very obvious due to the poor editing.
Revealing mistake: When Buffy is walking alone into the desert, a man's legs are visible in the upper-right corner of the frame and remain visible for a second or two. (00:03:35)
Revealing mistake: When Glory creates the demon snake thing, we see a CGI image of the creature, but Glory and Buffy are nowhere to be seen. When we are shown where they are standing, their position should have been visible when the snake demon was created. (00:27:05)
Revealing mistake: When the porter is dragged back onto the train at the start you can see (more obviously if you watch in slow motion) that there's absolutely nothing grabbing him, not even someone's shadow. (00:04:55)
Revealing mistake: When Spike, Xander, Anya and the Buffybot are attacked by vampires in the cemetery, the grass for the cemetery moves underneath the vampires' feet as if it were a carpet. The cemetery was a sound stage and not an actual cemetery.
I Was Made To Love You - S5-E15
Revealing mistake: When Giles pushes Spike into the cabinet at the magic shop, you can see Spike's reflection in the glass of the cabinet's doors. But Spike is a vampire and therefore should have no reflection.
Revealing mistake: Watch the shot of Glory as she is hit by the truck at the beginning of the episode (easier to see in slow-mo/frame-by-frame). She isn't hit by the truck at all. It goes right past her. A simple optical illusion used in filmmaking for decades.
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 ★