The one that immediately springs to mind doesn't exactly fit the criteria, because it wasn't even true at the time that I was taught it in public school in Texas. But my history teacher taught me that no real historian called it the "American Civil War," and that it was correctly called "The War of Northern Aggression." And, of course, although the Confederacy did want to keep slavery legal, their actual central reason for seceding was "states rights."

Like I said, both of those are simply lies. Only propagandists call it "The War of Northern Aggression", and it was always explicitly about slavery.

The sad thing is that I believed and repeated these lies for years after that. Note that, like most people, I didn't have access to the internet to easily check things myself. Since at the time I had zero interest in reading about history, it was difficult to correct my knowledge.

It has demonstrated, to me at least, the importance of keeping propaganda away from children. The more you lie to children, the harder it will be for them to become functioning adults.