Every once in a while i'll "bump" my scale pan on the cradle and the 1/10 will jump a groove , I try to be careful but it happens.
As mentioned these are delicate instruments and a slight tap or bump will make them move, also if I haven't used the scale in a while I find that "gun room" dust will settle in the grooves ( raising the poise, not much but enough ) so a clean soft toothbrush takes care of that.