Warning: require_once(): Unable to allocate memory for pool in Drupal

Warning: require_once(): Unable to allocate memory for pool.

This error is related to the APC extension in PHP which is located here: /etc/php.d/apc.ini

Go ahead and edit that file, by increasing the memory resources values and restart your web server

Useful Links:
http://www.cyberciti.biz/faq/linux-unix-php-warning-unable-to-allocate-memory-for-pool/