I realize that desktop boards are all at end-of-life so I'm not expecting any official assistance. I am posting this in case anyone has seen this behavior and can shed some light on it. This happened on a system with i5-2500K, DZ68BC (BIOS of 0028), 16 GB of Crucial RAM, and a Velociraptor system drive. I have the OC set to Automatic which gives a slight boost to 3.8.
This morning I was going to use the system to boot a copy of Lightweight Portable Security written on a USB 2.0 flash drive. I have used this USB flash drive on another Intel system (DH67) so I know it works. I normally have network boot, UEFI boot, USB boot, boot USB first, and removable device boot disabled in BIOS. So I went into BIOS and enabled USB boot and boot USB first. However, I think I unintentionally enabled removable device boot. After pressing Save, I inserted the USB flash drive so it would be recognized after the BIOS screen disappeared. A bit later I saw an error I had never seen before, something to the effect that removable devices were bad, very bad. The two options it gave were not terribly clear and I chose one of them (the left one). I immediately got back into BIOS to see what happened and discovered that a super-F9 had been executed. What I mean by this is that every setting was reset to default. What was bizarre is that the supervisor password had been deleted and the tab for Performance was still active (it did not ask "Do you really want to change settings?"), but the settings were all reset to default.
I tried again later today. I got into BIOS and made darn sure I only enabled USB boot and boot USB first. Then just before I saved the new settings, I plugged-in the USB flash drive. Almost immediately the BIOS screen disappeared and the system rebooted. I got back into BIOS and saw that all of the settings were reset to default as before.
P.S. 0028 is the newest Sandy Bridge, but 0035 (Ivy Bridge) has bugs preventing OC. And it's not possible to revert back to 0028 once Ivy Bridge BIOS has been loaded. 0036 and 0037 may have fixed the OC issue, but if not, there's no going back. Courtesy of intel.com: "Warning: BIOS version 0035 includes support for 3rd Generation Intel® Core™ processors. Due to structural changes in this BIOS version, once a board has been converted to this version or later, it will no longer be possible to downgrade the BIOS to version 0028 or earlier. Attempting a BIOS downgrade to a previous version may fail – the board BIOS will remain at its current version."
UPDATE: Okay, no one was going to comment so I took N.Scott.Pearson's advice from another thread to only set "USB boot" in BIOS and use F-10 to select the device. That works. I also learned to never insert a USB flash drive unless the system is powered-off or after the system has completely booted and finished the logon process.