The first season of Boy Meets World featured the character Minkus, an effeminate nerd who, because he was smart and nerdy, served as an antagonist of sorts for the show's protagonists. The very last scene of the season features Topanga explaining the power of the mind, saying that if you want something enough, you can mentally will it into being. Cory and Shawn then look at each other, think really hard and make Minkus disappear. Like actually, he disappears into thin air.
It seems like a cute, non-canonical joke until you realize that after that Minkus is written out of the show. Minkus only appears once more in the whole series to discuss how he's been missing for the past four years. This implies two terrifying facts about the show: not only do Cory and Shawn possess god-like super powers, they also use them to will a child out of existence. This explains why Feeny keeps following the boys wherever they go: It's up to him to keep their horrifying powers in check. He's basically Professor X.
The season six episode "My Baby Valentine" revolves around the birth of Cory's baby brother, and his subsequent time in the neonatal intensive care unit. Though he is said to survive his illness, he does not play any significant role until one year later, in the episode "Brave New World" in which Cory has a long heart to heart with his little brother. Watching that episode you notice something strange, though: even though he was born only a year earlier, Joshua is now at least three years old.
Okay fine, it's a sitcom. We don't always know how much time passes from episode to episode, but here's the thing: The kid Cory talks to had appeared once before in the episode "The Honeymoon is Over," only there he was just an unnamed child who appears in Cory's hallway and speaks a single line: "I see dead people."
At the time, it seems like a then very current reference to The Sixth Sense. However, when you later find out that the boy in the hall is actually Joshua, the line becomes a clue to a hidden truth of the show: The boy actually does see dead people, because he himself is dead. Joshua actually died in the NICU, and now his creepy little boy ghost occasionally haunts his family.
One of the most notable aspects of Boy Meets World is its insanely nebulous continuity. Main characters disappear without explanation. The physical appearances of certain characters change from episode to episode. At one point, Shawn has siblings other than Jack who disappear without a trace. Don't even get me started on Morgan!
All this shifting continuity is enough to make the show feel sometimes like that episode of The Simpsons where Homer keeps time-traveling, and changing things, but nobody notices. Of course, this doesn't apply to Boy Meets World though, because it's not like anyone ever time-travels... except, oh shit, they totally do!
There are a shocking number of Boy Meets World episodes devoted to the topic of time travel, and while most of these are implied to be dreams, at least one actually happens. In the crossover episode, "No Guts, No Cory," Salem, the cat from Sabrina the Teenage Witch, sends everyone back in time. Nothing about the episode implies that it is non-canonical, meaning that all of those apparent continuity errors start making a lot of sense: Cory clearly went back in time and changed history, tearing a hole in the space-time continuum and causing the butterfly effect to wreak havoc on the world in which the show takes place. This totally explains why the actor playing Topanga's dad changes three times over the course of the show... yeah, that makes much more sense than just '90s sitcom laziness.