https://www.reddit.com/r/bindingofisaac/comments/3srcsc/we_fucking_got_it/
Here you have a video. It's in Spanish..... sorry this is what I have.
This things in the games makes me wonder if SC will have a mystery like this to unlock in the real world.
If you liked it there is another case of this with the game Trials Fusion (also in Spanish, sorry, if you find a great video about this mysteries post it here):