This problem happen in my VPS after i install Alternative PHP cache (APC). And also i already have Zend optimizer installed on the same VPS. After i installed APC, my WordPress blog show strange behavior. Sometimes i got Error 500 Internal Server Error, and sometimes i got PHP fatal error.
Here is the error messsage:
Fatal error: Cannot run code from this file in conjunction with non encoded files in /home/xxxx/public_html/wp-includes/compat.php on line 66
As i try to find the solution on Google, there are lot of others complaining the same thing. You may need to read the details on faqs.org as it clearly state that APC doesn’t work with Zend Optimizer.
So the solution i have to choose one of them. And i choose APC since it need by W3 Total Cache for my WordPress blog. Since i have WHM root access, i turn off Zend Optimizer from EasyApache (Apache Update), and recompile everything to make sure Zend Optimizer removed. Now my blogs working normally again.