Open ./user/classes/external/update_user_device_public_key.php in your preferred editor insert a deliberate syntax error, for example change the class lin eto the following: class update_user_device_public_key extends external_api child class ooops I made a mistake { Run the following query to insert bogus data into your { external_functions } table: INSERT INTO `moodle`.`mdl_external_functions` (`name`, `classname`, `methodname`, `component`, `capabilities`) VALUES ('mod_invalid_get_foo', 'mod_invalid\\external\\get_foo', 'execute', 'mod_invalid', ''); Navigate to Server > Web services > API documentation in site administration Confirm the page loads without exceptions Confirm presence of an error, which reads as follows: Coding error detected, it must be fixed by a programmer: Cannot find file /externallib.php with external function implementation