On very busy sites you may notice more or less warnings about potential cache slams in your error logs. Exceptions are thrown in the code if this errors occur and the output in the front end may not what it should be.
The problem is a configuration option in APC that was added in good intention but results in a bad behavior of the system. The PHP documentation contains some background information about this option, which is deprecated since APC 3.0.11 but still enabled by default. To prevent those problems, make sure that the slam_defense option in your php.ini file is turned off:
php_admin_value apc.slam_defense 0
Please make sure that your are using at least APC 3.0.11 which uses a write lock instead to prevent cache slams. Earlier versions will cause trouble as there will be a race for caching the PHP files by the processes of the web server if the web server is restarted.