Due to the huge amount of data stored on our servers, and the fact that even the best hardware, databases and filesystems are capable of corruption, sometimes routines will need to be run that correct such errors. For the period in which this is happening there may some necessary downtime. All data is mirrored in two places automatically in case of hardware failure, as well as there being regular backups.
We have our own utilities for repairing damaged tables in MySQL and Postgres databases, and we keep (but don't guarantee) complimentary backups which can often be used to restore a database to a prior uncorrupted version. However, we encourage our users to make regular backups, so that there will be no danger of data being lost.