I encountered this on CentOS 7.2
A server had a bad entry in /etc/fstab.
This wasn't critical for the operation of the Linux OS (i.e. it wasn't / , /var etc), just a data filesystem.
In this case, LVM hadn't quite been configured for datavg, but the filesystem was listed in fstab
/dev/mapper/datavg-data /data xfs defaults 0 0
When attempting to boot, it stopped when attempting to mount filesystems and forced me into the emergency shell (rather than booting all the way so I could fix it in there). No details on what specifically was causing the boot failure.
Lesson learnt, make sure fstab is always accurate, even for non-system filesystems