Revealing mistake: When Doug stops Pacey on the street to talk to him, Pacey turns to face Doug (and the camera). You can see at this point, the microphone sticking out from Pacey's upper shirt. (00:04:25)
A Family Way - S4-E5
Revealing mistake: During the scene when Dawson and Gretchen are talking on the bridge, two couples walk by in the background. An older couple passes from left to right, and a younger couple walks from right to left. The younger couple is a woman with blonde, curly hair wearing a green top and beige/white skirt, and a guy wearing a green or blue top and white pants. During the same scene, the same younger couple again walks by in the background from right to left two more times. Then the older couple walks by again from left to right. Then one final time, the younger couple walks by yet again for a fourth time from right to left. Guess they were using the extras a lot that day. (00:27:15 - 00:28:50)
Coming Home - S4-E1
Revealing mistake: This scene of Dawson and Joey in the parking lot of the "Dive-In" was not really filmed in the same location. The show only had the location of the "Dive-In" for a limited time, and had to film all other related scenes in the parking lot of the studio in Wilmington, NC. Any scenes that don't show water in the background; and are just palm trees, lighting etc. were actually shot several weeks later in the parking lot, which was recreated to be part of the beach at the "Dive-In". (00:33:30)
Revealing mistake: When Dawson arrives home after his evening with Gretchen and friends, his Mother is seated on the couch waiting up for him. As she talks to him, she stretches back, still seated on the couch; and you can see on her stomach the outlines of the padding she is wearing to make her appear pregnant when she really isn't. (00:35:25)
Mind Games - S4-E16
Revealing mistake: When Jen and Jack are having coffee at the coffee shop, Jen walks outside and turns to bang on the outside window of the store to get Jack's attention. When she hits the window, you can tell it isn't a real window made of glass. It's obviously some sort of plastic, as it flexes too easily and moves too much to be a real store window. This was probably done to make the window safer for the actors during this scene.
Revealing mistake: When Grams is in the Kitchen with Jen and Jack, her sweater isn't buttoned right. When she walks close to the table you can see that the bottom edges of the front of the sweater don't match. It appears to be about one button off-one side is higher than the other. (00:26:35)
Coming Home - S4-E1
Revealing mistake: In the first scene, Pacey says something like see I think we should probably drop anchor here, have a bite to eat (and other things)... As he's walking around the boat, the camera shifts over and you can see two ropes that indicate the boat is already anchored or docked to something.
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.