Fear nothing except bad backups. Even then, your tools may fail. I am an optimist in all things, excluding IT. I always attack these things with the knowledge they can and will fail. I don't think I've ever heard of any system anywhere that has never proved without errors ever. Have a backup for your backups.
I've personally had a 10 disk RAID5 array fail, disk after disk, due to manufacturers defect - they came from the same batch from the same plant.