Using version 0.9.6.1 I am seeing an issue where "half on" cached pages are not being removed after their expiration time. With the default expiry at 3600 seconds, I have cached pages still being served after five full days! wp-content/cache is owned by the web server process and both it and all the contents of the directory are writable. Even after setting the expiry to 1799 seconds, the expired pages are not removed. Using the "Delete expired" or "Delete cache" buttons works fine to remove the expired files, suggesting there isn't a permission problem in the first place. Any ideas where I start looking for the source of the problem?