With Linux 3.14, you might notice in /proc/self/mountinfo that your root's parent FSID is now 0, instead of the 1 that it's been for the last N years. +Tejun Heo
wrote the change (9e30cc9595303b27b48) that caused this, but the change comes in a rather innocuous way. Instead of an internal kernel mount of sysfs being assigned 0, it's now the initramfs.
So far, this has already caused switch_root and findmnt (from util-linux) to break, cp (from coreutils) to break when using the -x flag in early userspace, and it's also been pointed out that systemd's readahead code makes assumptions about a device number of 0.
Are we now supposed to go and change all the assumptions in userspace about 0 being special? I'm conflicted. The kernel isn't supposed to break userspace, but it seems to me that FSIDs were never something to rely on -- similar to the block device numbering scheme.