This is a simple test case:

    $ bin/spring test ./test/unit/person_test.rb ./spec/models/person_spec.rb
    # Run options: --seed 28283

    # Running tests:



    Finished tests in 0.068785s, 334.3730 tests/s, 0.0000 assertions/s.

    23 tests, 0 assertions, 0 failures, 23 errors, 0 skips

Total meltdown, with no explanation.  This could be triggered by a buggy `Guardfile`, e.g.

    # ... other definitions snipped ...

    def all
      all_testunit + all_specs
    end

    def cucumber_guards
      watch(%r{^spec/support/(.+)\.rb$}) { all }
    end

    group :minitest do
      guard 'minitest', guard_opts do
        startup_guards { all_testunit }
        test_unit_guards
        all
      end
    end

    group :rspec do
      guard 'rspec', guard_opts do
        startup_guards { all_specs }
        rspec_guards
        all_specs
      end
    end

If the bug isn't glaringly obvious at first sight then that reinforces my point ;-)  Input validation is a good thing for users.