After a crash, two of the arms on my 3DR Hex shifted slightly. Enough that the props overlapped. I didn't notice it for a while, but on a subsequent flight, they prop tips touched. The hex didn't crash but the noise it made certainly got my attention.
My solution was to add more sonar mounting brackets to beef up the arm placements. It also gave me more places to mount stuff. (Copter is upside-down in the photo).