Mac OS X 10.11
When attempting to open a remote file, I'm confronted with what I assume is supposed to be a dialog box asking for my user name and password. However, since it's wall-to-wall black (except for tiny square bites taken out of each of the 4 rounded corners), I have no way of knowing whether I'm even in a field that I can type in. There are apparently active buttons on this all-black window, because clicking here and there within it opens additional all-black windows of different sizes in the same vicinity.
I also get windows of the same all-black-with-white-corners appearance in what I'm guessing is a progress report when navigating to a layout where some sorting needs to take place. These eventually go away when the sorting is complete.