Its not really an exploit but it is unintended.
currently if you re-queue for headless horseman after killing the horseman from inside the instance it just summons more people into the already completed event and when they leave they will get dungeon deserter.
i assume this is because its still a normal instance and the lfg assumes you need to fill slots to finish the dungeon not the event previous to that it used to recognize it was an event and it was finished so it would queue you to a new one.