Originally posted by jonnin:

new pieces can get confused and change color slowly, eventually busting.
I had one of these in my castle and adding a supporting bit resolved it. Before I put in support it would vanish when I opened the game or came back after a long time away -- never could catch it in the act and the rocks it broke into were hiding inside walls.

These were not new pieces. They have been there forever. I did not modify anything on that building.