I had a strange one on a system running Solaris 8 yesterday.

Set up mirror root with SDS and rebooted. Got system to OBP and did:

nvalias root disk0
nvalias mirror disk1

devalias confirmed these entries

Then did

setenv boot-device root mirror

Firstly it wouldnt let me set the boot-device so that it had two entries like it was when i started (boot-device disk1 disk1:a net) and secondly after the reset all the changes were gone, so it looks like it wasnt saving it to the NVRAM.

FYI use-nvramrc? was set to true.

Dave