What happened to me in minecraft?

I was playing minecraft pc and i decided to go in a cave when my game started looking different the cave was surrounded with black blocks and i saw pictures of items in minecraft like a diamond pick and wooden shovel. And on one of the blocks i saw herobrines face. Now i freaked out and destroyed the world input the same seed and thought it was a bug but it happened again only that all of the blocks had herobrines face so i decided to stop using the seed this happened around update 1.4.7 i did end up using it in 1.5 and it was gone but i still wonder what happened. Know there are no videos about it and i don't expect you to believe me but it really did happen and it would make my day if you guys tell me if it was a glitch or a bug. And before people say did you have a mod installed no i did not. Hope someone tells what it was and thanks