I sometimes get the following error message "Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 8 bytes)
If you work in localhost, change memory_limit in your {php_installation_dir}/php.ini to a higher one.
Otherwise ask your web hosting service to increase the memory limit of the server.
(You can check your PHP memory limit in your Drupal site too: /admin/reports/status)