quotes from core/email_api.php
line code
+-247 $t_bugnote_date = bugnote_get_field( $t_bugnote_id, 'last_modified' );
+-249 $t_bug_date = $t_bug->last_updated;
+-375 # check that user can see bugnotes if the last update included a bugnote
+-376 if( $t_bug_date == $t_bugnote_date ) {

Why is this? If the last bugnote was added 10 minutes ago and the bug hasn't changed since, the check would still return true.
Wouldn't it be better to check against time() and time() - 1 ?