I broke the sign and it blew up my room. I was on peaceful and there was no TNT anywhere. No fire (although that may be due to the moat that flooded my room when it exploded) appeared and the floor was not destroyed at all. The explosion almost traced the exact outline of the room. I posted on the Minecraft forums (under the name bunny9starry) and nobody had heard of it before. A few days later, I rebuilt my room and broke the sign again and it exploded for the second time. I did not manage to get a screen shot of the original room, but I got one of the exploded room and the rebuilt room. It was on a private world.