During World War II, the Army Corps of Engineers wanted to hide the Lockheed Burbank Aircraft Plant in case the Japanese decided to attack the West Coast. So, of course, they built a fake subdivision on its roof.
Using camouflage netting and trompe l'oeil, they brilliantly disguised the enormous plant as something completely benign. Just look at these before and after pictures! Fortunately, it all ended up being unnecessary, but what a cool example of creative thinking in a time of crisis. [Think or Thwim, Thanks, Jason!]