Plot hole: In 'Graduation Day pt 2' the Mayor has his vampires stand at the bottom of the steps leading out of the school to stop any escaping students. The thing is, all the parents managed to escape out the side exits. Since the Mayor was expecting all the students to try and run too, why didn't he have these exits covered? (00:33:15)
Plot hole: When Buffy enters the house to rescue her mother she uses a stake to stop the front door from closing. A vampire then grabs it and the door closes. Why did Buffy use a stake knowing that there was a vampire there when she was almost killed by a vampire with her own stake earlier in the episode because Giles had been injecting her with a muscle relaxant?
Plot hole: Healing Angel requires draining the blood of a Slayer. Buffy tries to kill Faith in order to get some of her blood but Faith gets away after Buffy stabs her at the end of "Graduation Day Part 1." The amount of blood on the dagger that she used wouldn't be enough blood to heal Angel, but it'd be a start. So why didn't Buffy just use some of the blood on the dagger? In the very first shot of "Graduation Day Part 2" we see the dagger coated with the stuff.
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 ★