A few years back, my main PC was running Windows 7. The OS was actually running well, but over the years, had built up a lot of cruft. I tend to use my home PC for mucking around with different packages, so it builds up a lot of crap.
To combat this, at the time, I wiped and re-installed Windows every few months. While I do currently back up the important stuff online, I didn't at the time, just relied on the fact I had multiple drives on the machine, with the OS being installed on an SSD, and applications and data being stored on separate HDDs, with my various network shares all stored on one dedicated drive.
Just as I had got to disk management page on the Windows 7 setup, my housemate walked in and started talking to me. I happily hit the "delete" and "new partition" buttons, thinking I'd selected the SSD. Then realised I suddenly had 1TB of unpartitioned space (the SSD is only 256GB). After a couple of seconds, I realised I'd just wiped one of the HDDs. Thankfully, it was just the network shares, which did contain some things I needed, but nothing I couldn't download again or otherwise re-create.