Filesystem full, but df says not.

[bob prohaska]

An rpi2 running -current reported errors during boot like this on the
serial console after a graceful reboot:

UFS /dev/ufs/rootfs (/) cylinder checksum failed: cg 0, cgp: 0x4c0a5f41 != bp: 0x38b82866
UFS /dev/ufs/rootfs (/) cylinder checksum failed: cg 3, cgp: 0x58e2c1f5 != bp: 0x903c297
UFS /dev/ufs/rootfs (/) cylinder checksum failed: cg 0, cgp: 0x4c0a5f41 != bp: 0x38b82866

/: write failed, filesystem is full
cp: /etc/motd: No space left on device
.
Mounting late filesystems:.
Dec 14 10:08:56 www kernel: pid 1394 (cp), uid 0 inumber 53912 on /: filesystem full

Root is on the microSD card, /usr /var /tmp and swap are on usb flash.

Nevertheless, it reached multi-user and allowed me  to  ssh in and run df,
which reported
Filesystem             1K-blocks     Used    Avail Capacity  Mounted on
/dev/ufs/rootfs          1473116   479936   875332    35%    /
devfs                          1        1        0   100%    /dev
/dev/msdosfs/MSDOSBOOT     51140     7588    43552    15%    /boot/msdos
/dev/da0e               52221244 28697844 19345704    60%    /usr
/dev/da0d                3044988   517860  2283532    18%    /tmp
/dev/da0a                2031132   122868  1745776     7%    /var

Still, any activity that wrote to disk repeated the filesystem full error.

This happened with three different kernels, dating Dec 12, 7 and Aug 26.
Running fsck -fy once in single user didn't seem to help, although it 
finished without obvious errors. Running fsck -fy repeatedly in single-user 
seems to have cleared the error, but it's a surprising development.
 
Thanks for reading,

bob prohaska