I used to be the system administrator for a small company. At one point, a hard drive controller failure led to data being written to the wrong location on the drive, overwriting other data. Unfortunately, by the time this was discovered, it had been going on for two weeks. The backup system hadn't reported any errors, as it was making a faithful copy of already-corrupted data. Once the hard drive controller had been replaced and the disk reformatted, I had to restore from backup, check the results on the hard drive, and then repeat the process with the previous day's backup if the data was still corrupted. Finally, with two-week-old backups, I was able to restore uncorrupted data. Then, every transaction that had taken place over the last two weeks had to be re-entered into the computer, while making sure that we didn't send out duplicate data to suppliers and customers. It took a month of hard work by all of the office staff to get the system totally caught-up again.
So, just because the backup system worked correctly doesn't necessarily mean your data is good.