Most videos and stories saying that they prove Herobrine is real are either
A: People making weird things in their SP worlds and recording video of "finding" it for attention
B: Actually made in multiplayer with a guy with a Herobrine skin , or
C: footage of the Herobrine Mod.
If you have seen strange things in your world and you didn't make them, (No lying, people) then your computer may have been hacked. I've seen some mentions of a Swedish hacker calling himself Herobrine who hacks Minecraft SinglePlayer worlds and makes structures like these, but I have yet to confirm that suh a hacker exists, or if he has been caught. If he is real, chances are he has access to Mojang's purchase lists. If my theory is correct, then here's a suggestion for victims of this hacker: Don't save your wolds to any one computer. Use a flash drive or other removable media storage device to store your worlld save files. I would guess that your account could only be tracked to the computer you bought minecraft from, so this should protect your worlds. Also: run a spyware check if you can.
