Under special circumstances, mouse dragged events are not passed to right component. This happens in Swing's support for MDI, when you work with JDesktopPane and JInternal frames. When JInternalFrame doesn't have focus (isn't selected) and user moves mouse over it, press left button and drags, mouse dragged events are passed to the glass pane of JInternalFrame, but not to the real content components. I'm attaching test case.

How to reproduce with test case:
1) Run testcase, destop pane with two internal frames will appear
2) Select "empty" (blue inside) nternal frame
3) Move your mouse over second frame, press and hold left button and move mouse pointer
4) See mouse dragged events passed only to glass pane

That means that if you have some component which reacts to mouse dragged events, it will not be notified correctly in this situation and can't signalize and perform some drag operation