I noticed last night that my syslog file was full of messages similar to this:Nov 7 12:57:59 tripel kernel: [ 3950.210383] EDAC amd64 MC0: ExtErr=(0x8) F10-ECC/K8-Chipkill error
My first thought was "Uh-oh... looks like a DIMM is starting to fail."
After running Memtest and much Googling, I've determined that the memory is most likely OK after all. The reason these messages started showing up just recently is a cooperative screwup (Asus and Ubuntu):
1. Ubuntu 8.10 apparently did not log ECC errors correctly (I just upgraded to the new 9.10).
2. The old BIOS in my Asus M3A78-CM (I think it may have still been running the original BIOS) was apparently causing spurious ECC errors to be reported.
So it looks like Asus screwed up, but their screwup was being masked by Ubuntu's screwup until I upgraded to 9.10. I've probably been getting spurious ECC exceptions all along, but didn't know it because Ubuntu wasn't logging them properly.
So I updated the BIOS... and as best I can tell, things are functioning properly now.