Lock nuts are used on the aileron hinges of RVs, there was a problem with them not being done up tight, which allowed slight movement of the threaded part of the hinge to flex the aluminium, resulting in crack in the aluminium.
It is easy when building to leave nuts loose, because you are doing a trial fit and then leaving it all as is, resulting on problems. No matter how careful you are when building these things can happen, even with double checking.