This has happened a few times now, and is really annoying.
During a Community Feast (especially if in a raid group, even if you are the leader) you will sometimes be shifted to a different shard. Since the Community Feast is already a PITA event to get to Legendary, getting shifted from an almost Legendary shard to an Uncommon with a few minutes left is very annoying and makes getting the pet/achievement a big pain. I would have had this achievement several times now, but having to get lucky about it not being buggy is not a good experience.