Yeah, I noticed that already in terms of them being in 6th grade in Season 1, and said to graduate from high school in the year 2000 in a Season 1 episode, but they end up graduating in 1998 instead.
Oh well.
Yeah,
season 1 Cory, Shawn, and Topanga are in 6th grade. Eric is in 10th
season 2 Cory, Shawn, and Topanga are in 7th grade. Eric is in 11th
season 3 Cory, Shawn, and Topanga never mention what grade they are in. Eric is in 12th
season 4 Cory, Shawn, and Topanga are in 11th grade. Eric has graduated but didn't get into college
season 5 Cory, Shawn, Topanga, and Angela are in 12th grade. Eric and Jack are freshmen at Pembrooke
season 6 Cory, Shawn, Topanga, and Angela are in freshmen at Pembrooke. Eric and Jack are sophomores, and Rachel is unknown
season 7 Cory, Shawn, Topanga, and Angela are in college and it is never specified that they graduate, but by the end of the season it's very clear that none of them are coming back. Eric, Jack, and Rachel all graduate.
Not to mention that in season 1 Shawn has a sister named Stacy, Topanga has a sister named Nebula. Later Shawn has a half brother named Eddie. By season 5 it's clear that Jack is Shawn's only sibling and Topanga has no siblings.
In season 1, Topanga's mom's name is Chloe. In seasons 6 & 7 it's Rhinannon. Her dad is played by three different people and her mom is played by two different people. The actors for her parents change between the season 6 finale and the season 7 premiere.
Cory & Eric's little sister Morgan disappears for the first half of season 3 and comes back as a different, much older actress. Her first line is "That was the longest time out ever."
There is a door in the school set that constantly changes. In one episode it's the principal's office. In another, it's a bathroom. In another, it's a janitor's closet.
And so on and so forth. There are tons of them.