Failed at step EXEC spawning /bin/plymouth (Debian testing)
I also had this precise error today as the result of a debian wheezy to jessie upgrade.
The system failed to reboot despite no errors from "apt-get dist-upgrade". The final error output via "journalctl -xb" (or "-xd") was associated with "plymouth" (an application that I'd never heard of). But it turns out failing to reboot had nothing to do with plymouth, but rather a minor anomaly under an ancillary entry under /etc/fstab: change "auto" to "noauto" for a cdrom device (nothing to do with NFS) and then systemd will allow the boot. This is an fstab line which functioned under wheezy and fails silently to allow a reboot under jessie.
There was no error via journalctl associated with fstab. It was lucky web searches that lead me to this obscure solution.
Combining the previous answers, this issue does appear to be caused by invalid entries in /etc/fstab.
In my case I am running inside virtualbox and it was a shared folder I had set up to auto mount on boot that was the problem. In the other two answers it was the settigns for NFS or CD-ROM device that was the issue.
I would suggest that to troubleshoot, just comment out all non-essential lines in /etc/fstab and then re-add them one by one until you replicate the problem.
The problematic line can then be diagnosed and fixed. It is possible during the dist upgrade that things like Vbox shared folders, network shares or or other specialized file systems were not upgraded correctly.
I had the exact error today.
I installed plymouth but it did not change the result.
It was caused by a wrong nfs entry in /etc/fstab. After deleting that entry the error disappeared. I guess this horrible behaviour is due to the stupid systemd.