When did you last test your backups?!
The cause of unrecoverable data is, in most cases, human error. Even the best backup system in the world is configured by a human being.
A backup system that was correctly configured initially, may run into an issue without you noticing. Backup system updates, configuration errors and all manner of other factors my lead to something that no longer serves the purpose. Equally, a backup system that has never been tested could run for years without anyone noticing that it unsuitable.
With these issues in mind, it is critical that your backup systems are tested regularly to ensure that, if needed, you would be able to restore your data.