When in overview, when you hover on any window, a X button (close button) is shown to close that window. But it is leading to a redundancy for the X button of window itself. See there is another X button (in the headerbar of window) visible under the X button provided by shell in overview for the hovered window. It kind of looks bad in the point of view of design.
What I suggest is that window controls in headerbar should be hidden when the user enters the overview. The window controls of headerbar are anyhow not clickable in overview (as they are part of the window itself and window is not clickable in overview mode (clicking only returns to window mode from overview mode))