I use PHP embedded in my Drupal content. This can cause problems with mod_security ("Internal server error", number 500).
The problem is that Drupal sends Edit content back to the server via the POST method. If your content has <?php> in it then it will probably get flagged as a security error.
If your hosting provider allows .htaccess overides then you can add lines to get arround the default rules. However, the security considerations are considerable. And as I have been recently reminded, I am not a security expert. Therefore instead of giving recomendations, here is the authoritative reference for mod_security.