An unexpected error occurred
Ideally, we don't want any errors, so the idea is to notify the viewer of an issue but maintain security.
You can turn that off under: Admin > Security here: admin.php?menu=admin&action=security
Hide PHP Errors [X] Hide all warnings and show "An unexpected error occurred #" for errors - View error log >>
Also, note that CMSB will generally override your display_errors settings, but the errors will logged under "Developer Log". I recommend hiding errors and enabling email notification of errors for production sites.
Hope that helps, let me know any other questions!