On a whim, I tried the SPI transfer test program on another Redboard I had laying around. Interestingly, I could not replicate the SPI errors on that second board. So I tried a third board I had made myself with a bare Artemis module on it. I let that system run the SPI transfer test overnight. As of this morning, it had no SPI transfer errors in over 440 million transfers. That would seem to suggest that my original board has a processor that is misbehaving under rare circumstances. In my experience, that is an profoundly unusual occurrence, but maybe that’s what it is. I would be interested if anyone else sees any errors when running the same test.