I was young and only had a deskop, so all my data was there.
So I purchased a 300GB external usb drive to use for periodic backup. It was all manual copy/paste files across with no real schedule, but it was fine for the time and life was good.
Over time my data grew and the 300GB drive wasn't large enough to store it all. For a while some of it wasnt backed up (I was young with much less disposable income).
Eventually I purchased a 500GB drive.
But what I didn't know is my desktop drive was dying. Bits were flipping, a lot of them.
So when I did my first backup with the new drive I copied all my data off my desktop along with the corruption.
It was months before I realised a huge amount of my files were corrupted. By that point I'd wiped the old backup drive to give to my Mum to do her own backups. My data was long gone.
Once I discovered ZFS I jumped on it. It was the exact thing that would have prevented this because I could have detected the corruption when I purchased the new backup drive and did the initial backup to it.
(I made up the drive sizes because I can't remember, but the ratios will be about right).