BS - I believe in Herobrine.
First time I had an encounter was when I first started playing - and doing only Single Player mode. I randomly generated worlds, and then would keep switching, making new ones, etc.
One World I created... It was the strangest thing. It was a randomly generated world, but after about a couple hours of exploring in one direction, I came across some obvious "person" made structures and tunnels/caves, some parts FILLED IN WITH re-used cobble.
It freaked me out so much that I didn't go back for a while, fearing I'd been "hacked"? Somehow someone is messing with me?
It was all very strange...
