When Darla encounters Buffy, she is confused about how she is so strong, knows of their kind (vampires), and can fight so well. However, Darla has encountered at least 2 slayers in the past, which is later revealed on the Angel series. Darla would have known that Buffy is a slayer.
When Buffy and Giles are talking about Sunnydale, he hands her a series of books with the spine toward her. But, when she hands the books back to him, the spines are toward Giles.
When Xander is talking to the girl at the Bronze his hand is on the pole and off the pole throughout.
In Buffy's room the lampshade is crooked, but when shown in the mirror reflection during her conversation with Joyce, it is on straight.
In the movie , Buffy is a senior , but at the beginning of the show , she's a Sophomore .
Storming out of the Bronze to save Willow, Buffy tells Giles "Don't worry! One vampire I can handle!" She passes Darla, still playing the innocent schoolgirl while calmly seducing her next victim. Later in the series though, it is established that vampires have super acute hearing. This close, Darla should have overheard Buffy's conversation with Giles about vampires and been alarmed.
When Principal Flutie tapes Buffy's transcript back together, he clearly doesn't match any of the edges up correctly.