I recently installed Acunetix WP Security on my WordPress site. I used the "WP Info" tab to change the permissions of my readme.html file from 644 to 400. Then on the "Settings" page, I checked the "Empty the content of the readme.html file from the root directory" option. The result was this error displaying on every page of my site, both front-end and back-end:
"Warning: file_put_contents(/readme.html) [function.file-put-contents]: failed to open stream: Permission denied in /wp-content/plugins/wp-security-scan/res/inc/WsdSecurity.php on line 679"
The solution is to empty the readme.html file BEFORE letting Acunetix change its permissions. But it seems odd that the plugin would itself create the conditions that would cause the error, then display that error in a non-intuitive fashion. Just a heads-up!