I thought that Mars used to have water on it.
Oh Wait...
lol seriously though, back when I was in 2nd grade (late nineties) we used to go to the school library on friday and check out books to read for the rest of the week. I always checked out paranormal books (the old ones with Nazca lines, Nessie, etc.), Those "Scary stories to tell in the dark" books (lol we used to fight over those), and those "Eyewitness" books about various subjects.
So I snag the space book, and it just so happens we were learning about geological stuff at the time. As soon as I turned to the page with Mars on it, I saw all those lines going up and down, which looked like dried up streams, rivers, and whatnot. I told my teacher about it and what it meant, and she pretty much shot me down quick (forget the exact words of course...but I just know no one believed me at the time).
Fast forward 10 or so years later, and scientists are running around pointing at those same lines, saying the same things I got shot down about in elementary school:lol
Also on a lesser note, I really thought most of the stuff in those Weekly world news mags in the store were real.