Audio problem: When Abbie is trying to get Pacey's signature and says, "I'm collecting signatures," the movement of her lips doesn't match the speech. (00:15:05)
Continuity mistake: The first time Abby reads the "mystery letter" out loud, she says "But sex changes everything." The second time, she reads, "But having sex changes everything."
Answer: It's only explained once in the series. Jack says to Andie "I tried to start kindergarten without you", meaning Andie was smart enough to start school early.