During the development of one of our sites we used a test blog with real blog posts and we called it "Test Blog". When we launched we either changed the name to "Blog" or created a new blog and removed "Test Blog". When you search for certain blog posts, it will return duplicate posts, one with /blog/ in the url and one with /test-blog/ url that goes to a 404.
There is no instance of "Test Blog" anymore and there are no re-directs still in place. I have this to robots.txt but it still returns when you search: