I added a SAN LUN to a volume that turned out to be in use, and was storing our only copy of a 500GB (sic) backup database tracking 60k+ tapes. After laying out the partition on the volume someone noticed that the database disappeared, and we were able to revert the partition and get the data back. Ended up buying some pretty nice Cognac for shortening the life of few folks. No idea why they weren't backing up their database though.