There's a bunch of other things wrong with it. They ret-con Corey and Topanga's relationship hella towards the later season, stating that they were always "soulmates", yet Corey only first meets Topanga in season 1 in Feeny's.

Shawn's backstory is constantly changing. He supposedly has a sister, but never mentioned again. Virna was always depicted as his mom, yet it's later revealed she wasn't his real mom, but some random lady married to his dad that took care of him, his real mom was a stripper who died? His dad Chett, was supposedly violent and abusive to Jack's mother which is why they got divorced, and Shawn later remarks during the college seasons he was the same way, yet in the earlier seasons Chett is never like that, he actually works as a Janitor at their school to make ends meet, but mysteriously stops working as a janitor right after that episode.

Topanga's parents are supposed to be just as weird as her during the earlier seasons, but they seem pretty normal, and even uptight when they're first physically introduced in that Pittsburgh episode. Topanga says she has a sister Nebula, who is never mentioned again.

Eric goes through the most dramatic change. First, he's depicted as being 5 years older than Corey, yet when the show ends, he's only 1 year older, and in the same grade because he took a year off after high school. He goes from the normal, under-achieving, chick chasing dude, to this eccentric, wacky, caricature of himself. The Eric we see during the college seasons is nothing remotely similar to the Eric during the earlier seasons, and no mention of this is ever made. There's no explanation as to what the fuck happened.

There are a bunch of other little things, Morgan disappearing for two seasons. Mr. Turner's friend Eli, the other teacher who was a regular for two seasons, just disappears and never returns. Same with Mr. Turner and Minkus, though they're vaguely referenced during the high school graduation episode.

I think initially when it came out, Boy Meets World was just supposed to be another Full House like sitcom. But as the actors grew older, they wanted to target another audience, so they changed a lot of the characters and a lot of the history to do. What we get, is initially two different shows from seasons 1-4, which is very family friendly and your typical sitcom, and seasons 5-7, which is more mature, tries to deal with heavier topics and make the characters more complex, but is forced to change a lot of their history to do so.