When you iconify a window, enlightenment will change focus to another (non-iconified) window.
However, if you have a key binding that iconifies a window, and you use it twice quickly, you can have the second action occur before focus changes (especially if you have animated iconification on).
The second iconify will operate on the now iconified window, and it will try to maximize it for you. Unfortunately, all window decoration is lost (title bar/borders/etc..) and enlightenment basically loses track of the window. You can't move it, close it, type to it, etc.. You can only get rid of it by killing the process that owns the original window.