Not overclocking at all - CPU and sdhost driver both at default speeds.
I didn't save the whole dmesg unfortunately but I did scan right through it and didn't see anything else relevant beyond what I posted.
The dd commands I had run were:
dd if=/dev/zero of=10GB bs=1M count=10240
dd if=/dev/zero of=8GB bs=1M count=8192
dd if=/dev/zero of=600MB bs=1M count=600
dd if=/dev/zero of=250MB bs=1M count=250
dd if=/dev/zero of=100MB bs=1M count=100
sudo dd if=/dev/zero of=/boot/300MB bs=1M count=300
sudo dd if=/dev/zero of=/boot/30MB bs=1M count=30
The first 5 were on the root file system and didn't experience problems, the last two were on the boot partition, the first of which - 300MB was a typo that caused the partition to run right out of space as there was only 58MB free. This seemed to be when the errors occurred but it could have just been a coincidence.