http://mathwithbaddrawings.com/2015/08/19/the-smartest-dumb-error-in-the-great-state-of-colorado/
The sign featured in this post by Ben Orlin is just up the hill from where I sit, and I think of it as an elaborate explanation of what I once heard from a veteran math teacher: "If you have 2 dogs and 3 houses, you don't have 5 dog houses."