Right I have a story!
Me and my friend Oliver were recording a youtube video when we ventured into a dungeon on a custom map. We had no mods or texture packs installed at this time, all we had was a map. We began to descend the stircase into the dungeon when i fell into the darkness! The next thing i knew I was under attack... by Herobrine! i was in such a panic a ran for the stairs, but we caught it on video! I don't know what happened or whether it was a bug but there is proof!