Running your rails application you'll experience this error at the very start of your development.
This is usually due to version shifting. As of Rails version 2.3, the
application.rbfrom the controller has been changed to
application_controller.rb. Now if you run your application to a lower version of Rails, you'll surely encounter this error.
application_controller.rbare just the same and all you need to do is to rename the file depending on your version.