Found out today while queuing for random Heroics that if the group starts the "gauntlet" sort of event then the healer leaves and the group picks up a new one, then everyone in this new group will get the http://www.wowhead.com/achievement=5995/moon-guardachievement even if the new healer takes damage. I happened to queue into the event on my holy paladin as well and also got the achievement even though i took damage. Have fun before its fixed :]