When two gui elements (for example sliders) overlap, and you try to "click" on them (i.e. move the slider) within the overlapping region, the correct behaviour would be that the front-most element "receive" the mouse click, since that is the one you're visibly "clicking on". However, the opposite happens: the gui element which is behind is the one that you move.
This happens both in editmode and not.
At least on Windows XP.