I have one that was proven false, and then later re-proven true: the existence of the brontosaurus.
When I was in elementary school, we were taught that they existed, they were big, etc. Then, at some point while I was in college, I discovered that actually what we thought was a brontosaur was a brachiosaur or an apatosaur. And then, when my kids went to school and learned about the brontosaur, I discovered that actually, they did exist!
I have one that was proven false, and then later re-proven true: the existence of the brontosaurus.
When I was in elementary school, we were taught that they existed, they were big, etc. Then, at some point while I was in college, I discovered that actually what we thought was a brontosaur was a brachiosaur or an apatosaur. And then, when my kids went to school and learned about the brontosaur, I discovered that actually, they did exist!