In the DMSGuestbook plugin, your filter that you hook to the_content is doing the wrong thing.
Here: http://plugins.trac.wordpress.org/browser/dmsguestbook/tags/1.15.3/admin.php#L3069
A filter should never output anything with echo, it should only return the modified content as a string. The actual the_content function does the echoing.
So when you do this:
echo $content;
$post_id = get_post($page_id);
if ($_COOKIE['wp-postpass_' . COOKIEHASH] == $post_id->post_password || $post_id->post_password == "") {
include_once ("dmsguestbook.php");
}You break the guestbook page *if* any other plugin exists that also modifies the_content and which happens to run after your plugin.
You need to modify this function to only change the value of $content and then to return that value. If you echo here, then things break.
