Untested backups don't exist either.

There are several horror stories of people going to their offsite tape backups to restore some lost data only to find that their backups didn't work due to a malfunctioning tape drive.

Other stories about encrypted backups where the decryption key is lost (backup data is encrypted with a randomly chosen "session" key, session key is encrypted using a public key, no-one could find the appropriate private key to decrypt the session key.

etc...