tldr: This is just an unusual experience report. It wasn't anything I couldn't fix on my own; I'm just sharing a little story.
So, leading up to the holidays, I built myself a new computer based on an X6 1100T and the M5A97 EVO motherboard. I also got one of the swanky all-in-one water cooling kits to go with it in a Define R3 case, and wanted to do some modest overclocking. So with Prime95 in hand, I fire up AMD overdrive and find... all six cores running at ~3850 MHz? Stock? At idle!? Double-check task manager, yes it really is idle. Then I started up the Asus motherboard management software and see... 1.55V core? By this point in the machine's lifetime, it has gone through a Windows 7 install, played a few hours of Civ V, compiled some code, and has run some Debian Sid, all with no trouble. So the machine is at least stable enough to go through most daily tasks at those settings. But a torture test run of Prime95 led to thermal throttling of the CPU in just a few minutes. Under Linux, Prime95 destabilized the machine and rapidly led to a kernel panic.
I turns out that this motherboard had a HT clock of 233 MHz, with all of the voltage control sliders maxed out**, and with turbo core and cool'n'quiet disabled. I set the clock back to 200 MHz, re-enabled turbo core and C'n'Q, and dialed the voltage control back to normal. With those settings I modestly overclocked the machine by multiplier adjustment to 3.8/4.1 GHz for a prime95-stable system. I didn't even try to find the failure point, I just found a happy place and stopped there.
In the end, I'm happy with the machine I've got. But it was very strange to go through this. My first guess is that this is someone's returned motherboard after they did some of their own overclocking. Except that it was not marked as an open box SKU, and all of the packaging looked factory new when I opened it up. A friend mentioned that maybe the factory end-of-line test setup could have gotten cut short, such that the board was not reset to standard settings before packaging. I suppose that's possible, but I don't have any evidence to either support or refute the idea.
I'm not asking for help in any way, just curious if anyone else has a similar story to share.
PS: The only supported resolution for the GUI setup interface is not compatible with my monitor (a 2004 vintage 20" apple cinema display), so I had to lug the machine over to the TV to actually perform any of the aforementioned BIOS configuration changes. Others have complained about this problem on the asus forums, only usually in reverse: for them, it's the TV that doesn't support the resolution. Grumble, grumble.
** Voltage control on the EVO motherboard is not as simple as just picking a voltage, and it is not 100% clear what each of the sliders actually do other than, "Higher setting increases the voltage. Sometimes."