When using the Drupal 8.3 Bootstrap theme if you enable "Rebuild the theme registry on every page load" in /admin/config/development/devel your page may fail to load with the error:
The website encountered an unexpected error. Please try again later.
If you check the watchdog logs the following error appears:
Error: Class 'Drupal\bootstrap\Bootstrap' not found in include_once() (line 30 of web/themes/contrib/bootstrap/bootstrap.theme)
The error might not appear until a cache rebuild so there could be a delay between when you enabled the setting and when the error starts to appear. The solution is to disable the rebuild theme registry option.