The following command, when run as any user, will crash systemd:
NOTIFY_SOCKET=/run/systemd/notify systemd-notify ""
After running this command, PID 1 is hung in the pause system call. You can no longer start and stop daemons. inetd-style services no longer accept connections. You cannot cleanly reboot the system. The system feels generally unstable (e.g. ssh and su hang for 30 seconds since systemd is now integrated with the login system).