foreach($posts as $post) {
    // $id is defined on the first iteration but $post_id is not
    $id = $post->post_id;
    $post_title = stripslashes($post->post_title);
    $post_title = str_replace('"', '', $post_title);

    // Filter hook - the errors are being generated downstream from here
    // Note also the use of undefined var $post_id
    $post_title=apply_filters('roster_post_title',$post_title,$post_id,$roster_slider_curr,$roster_slider_css);

    // $post_id is defined after this point
    // ...
}