Hey Mattnumbers, this bug is a rare case where the game client doesn't load entirely correctly. It shouldn't have an impact on gameplay and fixes itself upon restarting the game client. If you have time, you can restart immediately upon noticing. If you're game takes longer to load, i'd recommend you just play it out.
Black boxes around Champion Icons and black Unit Icons on the Minimap
In addition to Creeps, Wards and Teemo Shrooms showed up as black squares on the minimap too.
http://i374.photobucket.com/albums/o.../Capture-3.png
Thanks for the report, and hope this helps!