If you have a tab strip with 2 or more tabs, if you left click on one to select it but hold down the left button and drag down to the tab content window you get the popup menu you would normally see when dragging it.
When the tabs been selected the same click should not causes the drag functionlaity to kick in.
This becomes a real issue when you have context sensitive tool strips.
See video, selecting a document removed a context based toolstrip which moves the tab up. The mouse cursor is now IN the tabs contents and the tab control considers this to be a drag operation so opens the menu.