Moons and Planets 
When I was in grade school we learned that Jupiter had twelve moons. We had a test. "How many moons does Jupiter have?" I wrote "12" and it was marked correct. In 1974 a thirteenth moon was discovered. The moon didn't just pop into existence in 1974, it was always there (at least when I took my test). Now we know Jupiter has at least 63 moons. The answer of 12 wasn't even close; what I was taught to be a fact was simply not correct. 
Now we have ten planets (or is it eight?) circling our sun. Is nothing sacred? What about "My very educated mother just served us nine pies?" What other "facts" from my childhood were incorrect. Are we sure we just have one sun in our solar system? 
Maybe that's why I like mathematics and theoretical computer science. Eight plus seven will always be fifteen; nondeterministic space will always be closed under complement. We know what we know; we know what we don't know; sometime what we didn't know we now know but nothing we knew later becomes false.
