Alas, I still work with a program where someone had the bright idea to play tricks like this and, as a result, you can get error messages stuck completely behind an unmovable topmost window.
Thankfully, there's one slightly obscure way to get Windows to move a window that's completely behind another window, but many people don't realize there's anything back there in the first place.
Right click it from the task tray and select move. Your mouse cursor will jump over to the top of the window (even if you can't see the window) and you will actually move the item you selected rather than the topmost window you appear to be clicking on.
I called it "slightly obscure" because while the move option is perfectly normal, I had no idea that you could still select a window that was completely behind other windows.
EDIT: And no, "close" wasn't among the options for that error.
Thankfully, there's one slightly obscure way to get Windows to move a window that's completely behind another window, but many people don't realize there's anything back there in the first place.