From the first paragraph of the Introduction to Mikhail Golubev's Understanding the King's Indian (Gambit Publications, 2006, p. 5):
The author has used this opening with Black for more than 25 years. How I got started was slightly unusual. When I was 8 or 9, I played a training game with a friend from my chess club, Dima Novokhatko. After 1 c4 I noticed a certain weakening of the a1-h8 diagonal, and answered with 1...g6.
What?!
Most of us as experienced adults won't notice any such weakening, and he did as a youngster. Impressive! I wonder how many other GMs can tell similarly precocious stories.