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.
Once More, With Feeling - S6-E7
Continuity mistake: When Buffy jumps off the stage in the Bronze that Dawn and Sweet are on and starts dancing, one of Sweet's goons that is lying unconscious moves from the far left of the bottom of the stage to the far right between shots.
All The Way - S6-E6
Visible crew/equipment: A boom mic is reflected in the window of Buffy's house when she and Xander sit outside discussing his engagement to Anya.
Continuity mistake: After Buffy decides not to take the antidote to the demon's venom, we cut to a scene at the mental institution. She says she wants to go home with her mum and dad. As she says this her dad puts his hand on Joyce's left arm. In the following shot, Joyce suddenly has her right hand on top of his when it wasn't in the previous shot.
Once More, With Feeling - S6-E7
Continuity mistake: From the start of the second scene in the Magic Box, the doughnut and cruller on Xander's plate constantly switch places. (00:03:35 - 00:05:50)
Seeing Red - S6-E19
Continuity mistake: Shortly after Spike tries to rape Buffy, he returns to his crypt and drinks a glass of vodka. Clem then arrives to watch television with him. During Spike and Clem's conversation, the position of the bottle of vodka constantly changes between shots.
Visible crew/equipment: When the Scoobies are attempting to leave Buffy's house by using magic, we see a close up shot of the house's front door knob. None of the Scoobies are reflected in the door knob even though they're standing right in front of it. There is also a fairly large black object reflected in it. A film crew perhaps?
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.
After Life - S6-E3
Visible crew/equipment: When Dawn is possessed by the demon and breathes fire in the magic shop, in the bottom right corner of the screen in one of the shots, a metal tube can be seen blowing fire onto the set when it should be coming from Dawn's mouth. A hand can also be seen holding a blow torch, igniting the tube and then quickly pulling out of shot.
Seeing Red - S6-E19
Continuity mistake: When Buffy is fighting Warren at the end, there are several shots when it's clear that it's a stuntman playing Warren. The stuntman is taller and slimmer than Adam Busch, and the shape of his face is different, as is his hair in the front.
Other mistake: As Spike follows Buffy into the hallway by her front door we see Spike's reflection in the mirror. Vampires do not have reflections, so Spike should not have been visible.
Continuity mistake: During the opening of the season 6 episode "Grave," Willow's nose starts bleeding and she gets blood on her upper lip. Over the next several shots the blood consistently keeps appearing and disappearing.
As You Were - S6-E15
Continuity mistake: When Buffy receives her letter from UC Sunnydale it says her address is 1630 Crestview, this doesn't sync up continuity-wise with the episode The Body as she tells the 911 operator that her address is 1630 Revello. She didn't move so it's a definite mistake.
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)
Seeing Red - S6-E19
Continuity mistake: When Tara got shot, the blood on Willows shirt was spread out, but in the next shot, there were big clumps of blood.
All The Way - S6-E6
Continuity mistake: When a vampire kicks through a car window trying to hit Buffy, some shards are still standing in the window frame. The angle changes, and the shards are gone. (00:37:00)
Tabula Rasa - S6-E8
Continuity mistake: When Buffy and Dawn are discussing Buffy's new name choice Dawn's hair changes repeatedly between shots. First it's over her shoulders, then behind, she has a clip in one shot, then she doesn't.
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 ★