Revealing mistake: During the fight scene at the end when Buffy is fighting the demons you see her throw one of the demons off the platform. If you watch him fall you can see him land safely on a conveniently placed mat. (00:38:10)
Revealing mistake: As Willow is raising the satanic temple from the earth, we can see the support cables holding the temple upright from behind. This is plainly visible in the bottom right hand corner, complete with bright orange clasps on the temple itself.
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 Sid stabs Marc after he has transformed, there is no blood on the knife that he uses afterwards.
Beneath You - S7-E2
Revealing mistake: When Buffy throws Spike into the benches at the church you'll notice that as he is about to hit the floor there is a blue mat where he lands. (00:36:15)
Revealing mistake: When we see Spike and Andrew on Spike's motorcycle talking about Onion Blossoms, a filming light is reflected in Andrew's helmet.
Storyteller - S7-E16
Revealing mistake: When Buffy, Spike, Andrew and Robin go to the high school and find it decimated, Buffy is thrown to the ground and attacked with a hockey stick. On the second swing, a sound effect is heard and Buffy falls over. But the hockey stick never makes contact with her.
Life Serial - S6-E5
Revealing mistake: In Jonathan, Warren and Andrew's van, whenever they're watching something on the monitors fed from their security cameras, you can see that the footage is actually pre-recorded due to the fact that the footage is being played through QuickTime media player.
Revealing mistake: When the ubervamp is at the door of the Summers' home, you can see a black string attached to the inside of the door.
Revealing mistake: When Buffy is invisible and in the social office in "Gone", she is searching for the summer files, and you can see a thumb picking them up. (00:20:40)
Nightmares - S1-E10
Revealing mistake: The Master's artificial thumbnail is visibly falling off in a few shots of the opening teaser. (00:01:30)
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: When Buffy runs in front of the car you can see knee pads on her knees. You have to look quite closely because they are the same colour as her tights. (00:15:00)
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 ★