Just a quick note: Have you just removed a logical volume and forgot to remove its auto-mounting entry from fstab? Don’t reboot just yet then, because otherwise systemd will enter into its emergency mode instead of just trying to skip that mount at boot time. Probably not that big of a deal since the emergency mode is some sort of maintenance shell. Unless your laptop keyboard kernel module hasn’t been loaded yet…
… y helo thar, live USB distro, plz fix my system.
I’m naming this as #1, simply because I know that systemd will continue to amaze me.