At the time of the Prescott launch, we updated the BIOS of the SK8N to rev 1004. We double-checked all the memory timings, which looked good -- CAS 2-3-2-7, which was the most stable setting. The memory clock read 200MHz, so all was well and good.

Well, no. You see, when the SK8N BIOS reads "200MHz", it really means "200MHz DDR." The memory clock was really set to 100MHz. There's no standard way to specify this in the motherboard world, but I'd gotten used to seeing actual clock rates in most BIOSes. When you see "166MHz", you'd know that the memory setting was for 333MHz DDR, for example. But it turns out that Asus meant "200MHz DDR". You know what they say about getting too comfortable?