Example: I have a button that creates a window on layer "UI". I have a sprite that reveals an object on layer "Game" (below UI layer) when mouseovered. If the window is over the sprite, mousing over the sprite will reveal that object above the window. Thus, I'll see a random object obstructing my UI layer.
Example: I made a simple camera control setting based on clicking over the map and dragging. If I click this window and try to move it, I also trigger the camera function because the mouse is over the map, even though the window is in the way.
Example: I have a button that creates this window. I also allow the same button to destroy the window. If I move the window over the button to hide it, then click the button, the button still counts as clicked and destroys the window.