If you find this in your apache error.log: Premature end of script headers in index.php and get a 500 error on your wordpress blog, then you should check your suexec log. When it reaches 2GB you will receive a 500 error and the message: Premature end of script headers

You can find the log here: /var/log/apache/suexec.log or simply enter locate suexec.log

Also check your /var/log/suphp.log … if it is over 2GB large you will receive the 500 error!