Ultima Online's Atlantic server, to put it lightly. Mythic was forced to rollback the game (and players' progress) yesterday on Atlantic to a backup from two weeks ago. Atlantic is the highest-populated shard in the Western hemisphere.
The official announcement cut straight to the point: "Due to issues beyond our control, I am sorry to have to inform all of Atlantic that we had to revert to a Dec 5th backup. We have tried to retrieve a more current backup but we have been unable to do so due to corruption issues."
Mythic said that it will be compensating affected players with a month of free game time, although that's undoubtedly small consolation to those who have lost progress and acquisitions during this period. Apart from the short announcement, there has been no additional communication from the studio on the issue.
[Thanks to Brad for the tip!]