So my group was doing headless horsemen, and I accepted the quest. Before we were unable to down him I got disconnected from the server. When I appeared, I could see the loot when I looted him, but could not pick it up (I told a guildie in vent to tell the group not to loot it >.>). We were about to do it again, but oddly enough, I could do it another time! So then we did it, and....I could do it a third time! Was unable to test it anymore since it was late and I couldn't enter any more instances for an hour thing showed up.
In other words: D/C from the server (force through third party mechanics maybe, or you can try to log off) after you start the quest and you can attack him (I D/Ced while I was DPSing). Get back in AFTER the headless horsemen is looted. There ya go, infinite loot from then on!
There is a second idea: if it was a freak glitch and not consistent I think it only counts the daily when you actually loot the horsemen so: You can try to get someone to accept the quest, have that person leave the instance (will not get credit, and cannot loot) and send someone else in, rinse and repeat, and do it for everyone else.