The patient first presented with classic memory corruption symptoms, like random segfaults. Ran memtest. No errors reported, but a flurry of corrections logged in the BIOS. Pulled a pair of dimms, everything cleared up. Swapped slots too, so I feel confident blaming the RAM.

It did fall outside my expectation of how ECC works. One bit errors and three bit errors, but not two? Some access pattern that memtest strides don't hit? I didn't really need the extra RAM, so I just moved on without it.