I have had the exact same thing happen today. Never been a problem and all of a sudden it broke. Not that big, but still rather annoying.

The sconces on a structure of mine were suddenly falling off when i used a portal in and out of my area. They however, were all on the same lining. What also happened is that they were all planted on a... i guess some sort of chunk line? So the wall was on one chunk and the sconces were on placed on those walls but they themselves were hanging in another chunk.

You can see the lines pretty good when using the hoe for making a stone pathway.

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.

They don't break, max 50% durability loss, no more.