The Song Remains the Same (2) - S6-E2
Revealing mistake: When Jen and CJ are talking at Boston Bay University, and CJ mentions 'The Stand'; Jen suddenly gets up to leave thinking he is referring to religion. As she turns to walk away; you can see for a brief moment the real tattoo on the lower back of Michelle Williams, in the slight gap between her shirt and pants.
Revealing mistake: When Joey and Pacey are playing Battleship, there are a few close-ups showing their game screens. Although you can see several push-pins in each screen, signifying the areas of the game they have tried; there are no actual ships visible. It looks like they are playing the game with only the push-pins, and not any actual battleships.
That Was Then (aka Before and After) - S6-E16
Revealing mistake: In the very beginning of the episode, Pacey is watching TV and gets up to look out the window towards Hell's Kitchen. As he moves back from the window, he accidently bumps the kitchen counter/cabinets. An entire section of 'counter/cabinet' moves noticeably; revealing it to be a cheap, flimsily-built fake kitchen counter/cabinet instead of a more substantial, real one. This was obviously just part of a set.
Revealing mistake: When Joey hands the reporter the cup of coffee in Pacey's office, you can tell the cup is empty. It tilts to one side, and is behaving as if very light without any fluid in it. Joey pretends to pour cream into the cup, implying there is already coffee inside. But the way the cup tilts in the reporter's hand, it's obvious it's actually empty.
Revealing mistake: When Pacey and Rich are physically fighting in the office, there is a quick shot of a person who is supposed to be Pacey leaning forward over Rich and hitting him. However, if you look closely you can see that the hair on this person although dark brown, appears much longer and curlier than Pacey's current hairstyle. Also-just before that shot ends, you can see the longer hair swing away from his neck loosely on the right side. The next shot of Pacey shows his shorter haircut, so I believe the longer hair was actually a stunt man substituting for Pacey in a risky, physical scene. (00:40:00)
Revealing mistake: Towards the end of this episode, Joey comes back to her Dorm room and spots the envelope Eddie left for her. You can see her name written on the outside of the envelope, and it's supposed to be Eddie's handwriting. But in fact, it looks exactly like Joey's handwriting as shown throughout the series. When Joey is talking earlier to Professor Hetson and he hands her the exam booklet with her writing on the front, it's the same style of handwriting. During other episodes, we see what is supposed to be Joey's own writing (ie: her note to Dawson during the Series Finale that she leaves on his bed) which is also very similar. So in fact, what is supposed to be Eddie's handwriting on the note to Joey really looks more like Joey's handwriting, or the same person who really does the writing for Joey's character.
Goodbye, Yellow Brick Road - S6-E21
Revealing mistake: Watch Busy Phillips' hand in the bar scene where Audrey sings - she's very obviously not playing that guitar.
All Good Things... (1) - S6-E23
Revealing mistake: During the scene when Jen is telling Jack about her illness in the hospital room, she asks him to get into bed with her and talk. As he does that, Jen lifts her right arm to allow room for him. There is a tube attached to her right arm at the inside of her elbow, that is supposed to be going into an artery. But you can see that the end of the tube is actually pointing the opposite way, and sticking up through the tape pointing towards us. Obviously this was because they couldn't actually insert the tube into Michelle's arm for the scene.
...Must Come to an End (2) - S6-E24
Revealing mistake: Immediately after Jen dies in the hospital room, Grams wakes up and notices something is wrong. She stands up and checks Jen's pulse, using Jen's left wrist. You can see a clear small marking/line of some kind on Jen's wrist, and Grams slides her fingers directly over this mark to check the pulse. Apparently the mark was a cue for Grams to know where exactly to place her fingers in the scene.
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.