At last I seem to have found a consistent way of reproducing a problem that has been bugging me for months. Sometimes, when clicking on a quite different part of the screen, the note I last entered or edited jumps to a ridiculous position and has to be corrected with CTRL-Z. These 4 pictures illustrate. I correct a note in the bass clef from E to D, then I click in the first note position on the next bar (in fact on the next line). The note I had just edited drops down off the stave. Sometimes this mis-positioning occurs even when clicking on the scroll bar right at the side of the score.