If PHP error messages are showing anywhere on your site, then the PHP setting
display_errors is turned on;
WP_DEBUG is not the only way this can happen. It may be set in your
.htaccess file, or it may be in your server’s
I’m sorry that you are getting error messages like this, but ICS Calendar’s handling of reporting errors, as I’ve noted, is fairly routine. The only reason any of these error messages is ever getting displayed to users is due to the
display_errors setting which is incorrect for a production server.