I was mining in and around -1000,10,-2000 in Exploration. I was converting a lava lake to obsidian when I got to the edge of the cave. For some reason, a block of lava was not converting to obsidian. I looked and there was a sign there, deep in the cave:
I reads "Herobrine was here..." I mined the block above the sign and the redstone to get a better look. When I turned around and looked back, the block above
the sign now had a sign that said "Muahahahaha..." or something similar. Creepy.
If you go to take a look, it's somewhere at the bottom of the shaft. Sorry I didn't hit F3 before the screenie. Also, don't dig down
at -1000,y,-2000! That's a shaft all the way down to level 10.