Many years ago I inherited a Sun workstation to look after because I'd shown some interest in learning about it. I was fairly proficient in DOS, and had used unix a bit at university. It was the only unix box in the company, and had been installed with the new fangled Mosaic web browser and had a 4800 baud modem to attach to the internet, wow! People would login and spend 15 minutes doing this new email thing!
One job was to apply a bunch of patches to upgrade the system. The instructions said to reboot into single user mode, and make sure the /tmp directory was empty. I did that, and deleted all the files in /tmp. But I did "ls -la" and there were a load of files starting with a dot. So I did "rm -rf .*". After a long time, I wondered what it was doing. I ran "ls" and it failed, I forget, but it was clear the machine was f**ked. The rm command had traversed to .. and had deleted pretty much everything.
The boss asked why I'd not backed it up, because the machine had a floppy drive! I think it had an amazing 40MB drive, so I said it was impractical to feed 60 floppies in (720K), even if we had them. He wasn't impressed. He later bought an adaptor card to use with a Plasmon (I think) optical disk storage thing.
So, I ended up reinstalling the system from scratch.