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.
Audio problem: After the opening credits, we see a band playing in the Bronze. The lead singer's mouth doesn't seem to 100% match what he's saying. It's mostly right, but just seems a little "off." (Ex. There's a few very brief moments where individual syllables seem to not match, etc.) It's very clear he's simply lip-syncing to a pre-recorded track.
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 ★