An example of this which I think every gopher hits at least once is the use of an error struct instead of an interface in a return value:

    func returnsNilError() (int, *MyErr) {
      return 0, nil
    }
    // case 1: no other use of error, nil works
    {
      i, err := returnsNilError()
      _ = i
      fmt.Printf("err is nil: %v\n", err == nil)
    }
    // case 2: someone adds a couple lines of code before that call...
    {
      _, err := os.Open("/tmp/file")
      i, err := returnsNilError()
      _ = i
      fmt.Printf("err is nil: %v\n", err == nil)
    }
    // Wait, what? Someone adding a line of code before your code changes how your nil check works?

Playground: https://play.golang.org/p/-t1Hq8WMF8l

This, I think, is the reason everyone uses "error" in their return values in go, even though using a specific type (like "*os.PathError") would make it much easier to know what possible error conditions a function could have.