Originally posted by Nerevar:

check thier color with the hammer. if they turn deep red they can sometimes break as they lack stability.

note : wait atleast 2 minutes after placeing something for the colors to adjust for new pieces. otherwise they all show green at first. thier color slowly switches afterwards.

It's a bug, most likely has something to do with building on a chunk border, green pieces just break in certain spots. Have had it a few times now. 2 wall sconces breaking constantly and my harbor, my docks all had 1 piece breaking in the same spot. Every time it was cause it was build on a chunk border. Sconces could not be fixed, harbor for some reason needed 1 piece of extra support, even though it didn't do anything structural integrity wise.