The database we use was setup in a redundant configuration with a second hot stand by unit. Full backups on a weekly basis were existent.
Then data content became corrupted over a time of 3 weeks and due to the fact that only small amounts of data were destroyed it was discovered the first time after 3 weeks.
An assumed bug in the database software caused this problem.
It was impossible to use the hot standby unit, since the corrupted data was just mirrored. Using a 3-week old backup would have skipped 3 weeks of changes and additions such as new customers or billing data worth millions of Dollars.
The only solution was to repair the corrupted database to preserve most of the files and content which took several days and caused downtime for a lot of services offered.