Usually, we something happen in the application level with 500 error , application throw an 500 http error.
it might be happen on the live website, the result is un ugly white page with "The website encountered an unexpected error. Please try again later." error inside.
custom 500 INTERNAL SERVER ERROR page module helps you to customize the page and show a proper markup with the proper message on 500 error.
module link: https://www.drupal.org/project/custom_500_error