Repeated panic: ffs_checkblk: bad block -1 on rpi2

[bob prohaska]

Hi all,

The last few attempts to build world have failed with 
panic: ffs_checkblk: bad block -1
The latest incident was at r291652.

So far, it's been possible to restart the machine and
complete the build/install cycle, typically in single
user mode as a precaution. Once complete the reboot has
been uneventful.

The same failure has been seen on both disk and flash
backed machines. Later instances have been on the disk-
backed machine, simply because it is the only one with
serial console. 

The latest event is documented to the best of my ability at
www.zefox.net/~fbsd/rpi2/crashes/build_12_4_15

Is there any chance this might be a hardware issue? For the
moment I tend to think not, but am far from certain.

Thanks for reading and any guidance,

bob prohaska