Revealing mistake: When Jen goes to visit Henry at the hospital, he is eating a small individual-sized Jello. There is another identical Jello on the same tray, in front of Jen when she sits down to talk to Henry. You can see that the corner of the Jello lid has already been pulled up rather noticeably. This is probably to make it easier for Michelle Williams, the actress to be able to open the Jello quickly during her scene.
Revealing mistake: During this scene Joey is running from the wedding to find Pacey. There is a young couple dressed in light blue, standing on the left side of the scene. The man appears to turn his head and see Katie Holmes running towards them, and puts his arm around the woman next to him and directs her across the grass over to the right. They both sort of suddenly run over to the right in order to clear the way for Katie to keep running straight through the area. It's obvious they moved intentionally so as to get out of her way, knowing the actress was about to run by them.
Revealing mistake: When Joey and Pacey are talking in the car as Pacey drives them to the Train Station, you can see at least 3 lanes full of cars and a great deal of traffic behind them. But when Pacey suddenly turns right, off the main road and into the entrance to the Train Station; you can see that suddenly the main road he has just turned off of is almost completely empty of any traffic in either direction, except a truck that drives by at the last minute. It also appears to be a smaller road. Apparently the scenes with their dialogue in the car were shot with background video that wasn't the actual location.