More info:

1) It appears to happen only with one disk, an old Quantum 210S (SUN0207).
Other disks, either with or without the Quantum present, do not seem to
have the problem.

2) It doesn't matter what SCSI ID the Quantum is or whether parity is on or
off. It might be a termination issue, didn't have any way to check that
internally. It doesn't occur only when shutting down; I booted from the
other disk, ran a fsck on the Quantum, and got the error at the end of the
fsck when it was trying to mark the filesystem clean.

3) Once the error occurs, you will never talk to the Quantum again until you
power-cycle. However, other disks in the system continue to work normally.

   Since the Quantum was just my install disk and the 3/80 seems perfectly
happy with its permanent disks, I'm not going to worry about the problem.

   --James