Major Windows on top of doors cannot be opened.

[Grek]

To replicate: Open a door. Construct a window. Allow the door to close. Most doors will now entirely cover the window, preventing the window from being interacted with or removed. The presence of the window makes you unable to interact with the door, despite it being the only visible structure on the tile - clicking the door does nothing, nor does walking into it. Some of the newer doors allow a sliver of window to be visible (and thus removable), but most don't.

This would be a huge game breaking exploit, except you can still just throw something at the door/window and eventually smash it.