@cas I didn't have a deeper look, but can't you use EVENT_BUGNOTE_DATA?

@atrol, was my initial idea too but this kicks in after validation and processing.

The event is triggered before the note is stored in database.
Isn't this what you need?

Are you sure? My test shows different. If I use that Event I do get a message but the note is already in the DB.

Snippet from function bugnote_add

    # Event integration
    $t_bugnote_text = event_signal( 'EVENT_BUGNOTE_DATA', $p_bugnote_text, $c_bug_id );

    # MySQL 4-bytes UTF-8 chars workaround #21101
    $t_bugnote_text = db_mysql_fix_utf8( $t_bugnote_text );

    # insert bugnote text
    db_param_push();
    $t_query = 'INSERT INTO {bugnote_text} ( note ) VALUES ( ' . db_param() . ' )';
    db_query( $t_query, array( $t_bugnote_text ) );

My mistake, you are right. Reviewed my test and looked at EVENT_BUGNOTE_ADD.
Please close issue.