One day on my main Arch installation I created a container inside a directory, and “booted” into it by using systemd-nspawn. When I was done with it I decided to do a rm -rf / inside the container just to be funny. Then I noticed that my DE on the host froze and I couldn’t do anything. Then I realized that systemd-nspawn mounts some important host’s directories on the container, and I deleted those when I did the rm -rf /. I didn’t lose anything, but it was scary.