Comment 6 for bug 1887744

Sorry, turns out the /etc/systemd/system service file needs to be a symlink to trigger the bug. This will do it:

su -
cat <<EOF > /crashsystemd@.service
[Unit]
Description=Template for instance %i

[Service]
ExecStart=/bin/true
EOF

cd /etc/systemd/system
ln -s /crashsystemd@.service crashsystemd.service
systemctl daemon-reload
systemctl start crashsystemd

After the 'systemctl start crashsystemd' my terminal says:

Failed to start crashsystemd.service: Connection reset by peer
See system logs and 'systemctl status crashsystemd.service' for details.

and in the systemd logs I see:

Jul 17 23:20:33 jturner-desktop systemd[1]: Assertion 'u->instance' failed at src/core/load-fragment.c:4798, function unit_load_fragment(). Aborting.

Things are pretty broken now. 'systemctl' and 'reboot' commands won't work. Don't forget to delete the /etc/systemd/system/crashsystemd.service symlink before rebooting.