Continuity mistake: When Buffy is shown pulling the secret chest out of her closet with weapons against the vampires, the window behind her shows daylight, but when she steps out of her window it is fully dark. There is also a cut scene right before that shows it was twilight, but there was late afternoon sunlight shining through the windows of her room. It's not possible for the sun to go down that fast.
Continuity mistake: At home Buffy helps her mother open a crate. She tears off the cover and leaves it hanging by the side. Next shot the lid is back on. Not nailed anymore of course, but there was no time to put it back on.
Visible crew/equipment: When Catherine and Amy get switched back to their original selves, Catherine takes the fire axe that Amy is holding by using her powers. The wire used to pull the fire axe towards her as Amy holds it up and says "Mom! Please!" is visible.
Continuity mistake: When Buffy finds Dr. Gregory's glasses on the lab floor she places them on the table, from the long shot she places them at the left end of the table near a plastic tub with a white lid, with the glasses open. Then it cuts to a close up where they are in front of a book on the right side of the table and are closed. Then cut back to the long shot and they are in the original place. (00:10:25)
Continuity mistake: In the episode 'Teacher's Pet', when Ms. French is eating her bug sandwich, her sleeves go from being up in the long shots to being pulled down in the close-up of the sandwich. (00:24:05)
Continuity mistake: When we see Miss French make herself a bug sandwich, there are two, maybe three bugs left on her plate as she folds the bread. But when she eats the sandwich her plate is empty.
Continuity mistake: After Buffy finds Dr. Gregory's glasses at the start of Miss French's class and places them on the table, they move position twice throughout the rest of the scene.
Continuity mistake: When Xander chugs the martini Miss French gave him, the way he's holding the martini glass changes instantly between cuts. First he's holding it by the top, then he's holding it by the stem, then he's holding it by the top again.
Continuity mistake: After Xander chugs his martini, he transfers the glass to his left hand. But then, it's back in his right hand instantly between cuts.
Continuity mistake: When Miss French scoots closer to Xander and asks if he's ever been with a woman before, Xander hands go from being apart touching between cuts.
Continuity mistake: When Giles and Buffy are talking about how to kill the She-Mantis, around the time Buffy says "Slice and dice!", her hands go from down by her sides to up in front of her chest gripping together between cuts.
Audio problem: When Xander is talking to Blaine in the cell, Blaine is explaining what Miss French/She-Mantis does. At one point Blaine says "She ties you up, " but his mouth doesn't match the words at all. His mouth and the words match for every other line, so I'm assuming they decided to add that line in post.
Continuity mistake: When Xander thinks he hears someone at Miss French's house, she distracts him by caressing his chin. In the two shots facing Miss French, she's touching his chin with her LEFT hand but in the shot facing Xander she's touching his chin with her RIGHT hand.
Continuity mistake: When Buffy is telling Giles and Willow what she read about praying mantises in the book, she slams it shut and says "Ha!" The scene then cuts from a wide shot to a medium shot of Buffy, and the way she is holding the book changes slightly between cuts. (More specifically, her hand and the book rotate about 45° between cuts).
Continuity mistake: Near the beginning, Miss French picks up a little terrarium with the mantis in it, the position of the mantis changes instantly between cuts. First it's on the left side, then it's on the right.
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 ★