When I was about 9 years old someone gave me a copy of an Erich von Daniken book, which I inevitably thought was the wonderful thing I had ever read. However, doubts started to creep in after a while and eventually I worked out that it was complete nonsense - but for a while I really wanted that stuff to be true and got quite angry that people weren't interested in this wonderful truth.

That experience left quite a mark on me - I still have a tiny bit of guilt at believing that stuff for a short while, even though I suspect it probably made me much more level headed and skeptical when I got older.