Writing $_GET somwhere in a page will result in an error.

[max]

Writing the word $_GET somewhere in when editing a page, will result in an error when saving:

Forbidden
You don't have permission to access /wikka.php on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

If the underscore or the word $_GET is removed, it works fine.

[DotMG]

Seems (I'm 99% sure) like a mod_security bug!

Deactivate mod_security. Use search engines to find more.

[DotMG]

Try adding this code in your .htaccess :

<IfModule mod_security.c>
  SecRuleEngine On
</IfModule>

For further refinement, see  http://www.modsecurity.org

[DotMG]

Sorry!

I meant

 SecRuleEngine Off