Continuity mistake: The first night that the Gentlemen float through the college, they are on the 2nd floor one minute (all the room numbers start with 2), then they turn a corner and are suddenly outside room 118, a floor below.
Continuity mistake: Near the end, when Spike takes the cup of blood from the fridge, we can see that it is full to within a 1/4 inch of the rim. However, when the shot changes and he drinks it, he tips the cup almost 45 degrees before putting his lips to the cup and drinking. If the cup had been as full as previously shown, the blood would have begun pouring out long before Spike ever drank form it.
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 ★