Continuity mistake: When Jen walks into the Video Store, she is wearing a black top, red skirt and a black leather jacket that is completely open and unbuttoned. As she talks to Dawson, who is behind the counter, she says, "I'm working on a project with Pacey in a 'human growth and development' thing - is he here?" She is standing up, but leaning slightly forward against the counter and her coat is still widely open. But the next shot of Jen, as she says, "But he told me that he was working today," the black leather jacket is suddenly closed together as if buttoned, and she is leaning foward more on the counter. Then, after Dawson walks over to the movie rack and Jen follows him, her jacket is again hanging completely open and unbuttoned.
Continuity mistake: When Jack and Andie are using the computer with Ben's Instant Message on the screen, there is a close-up of the computer screen as they wait for Ben's photo to download. Just to the right of the photo is the entire IM, which shows all 3 lines of dialogue between Ben and Jack. The last line says, "So. What do you think?" But after the photo is finished, there is a sudden 'beep' from the computer signalling another incoming IM message, which Jack then reads out loud as if it is a new message. But what he reads is, "So. What do you think?", the exact same line that was already in the IM box before the photo finished downloading. The implication is that he is reading a message that just came in, yet that line of dialogue from Ben was already present in the IM box previously.
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.