In case you have problems with the custom error page for 404 or 500 errors you should create a .htaccess file in the public_html directory containing one of the lines below. If you already have a .htaccess file please add this line:
For custom 404 error page:
ErrorDocument 404 /404.shtml
For custom 500 error page:
ErrorDocument 500 /500.shtml
The above directive will apply recursively to all subdirectories as well.