QGIS » QGIS Application

In the layers pane (TOC) vector layers have one line for the layer overall and then one additional line underneath for each symbol that has been applied to the layer. The top line has a +/- to show/hide the symbology lines.
If the symbology lines are shown, when the "Apply" or "OK" button is clicked in the layer properties dialogue, they automatically become hidden (this is annoying if you want them to remain shown :) )
Similarly, if you right-click on a layer and choose "Paste Style", then the symbology lines become hidden. However, in this case, if the layers pane is full with the scroll bar visible, and it is scrolled right to the bottom, when the symbology lines become hidden, this layer also becomes unselected, and whatever layer is now in the same place that it was on the screen becomes selected.
This is potentially dangerous if the user does not notice that a different layer has become selected.