It was my beta BIOS G16. I bought some tweezers today, and moved the BIOS_SELECT jumper to force a boot from the backup BIOS, which was the shipping BIOS. Booted up the first time. Now that I'm paying closer attention to the boot codes, I can tell that the bad boot was showing boot code "dd", not "PP", since the orientation for "dd" is the same as "FF" which I'm seeing and recognize as a common boot code. Unfortunately, "dd" is not listed in the common boot code list so I don't know what was the problem with G16, which had booted up fine a bunch of times before the Explorer update reboot. Anyway I guess I'll run the shipping BIOS until I get around to doing an OC.