Used inside comments_template that allows you to catch all the comments going through the query for the post.
<?php function shuffle_comments( $comments , $post_id ){ return shuffle( $comments ); }
add_filter( 'comments_array' , 'shuffle_comments' , 10, 2 ); ?>
comments_array is located in wp-includes/comment-template.php.
comment_save_pre, pre_comment_approved, pre_comment_content, preprocess_comment, wp_allow_comment()