Here's an example of two duplicate servers, one backing up for the other, where all data was lost (destroyed intentionally, actually).
This was reported on Slashdot today; here's a link to the actual article referenced by slashdot:
http://news.bbc.co.uk/2/hi/technology/8049780.stm
(Some of *my* data was lost, as I was a member of this group about six or eight years ago. Inactive data, but data nonetheless.)