I've confirmed that version 2.5.13 will usually continue to work below 0°C, whereas 2.5.12 and before will go into "thermal shutdown" mode because -1°C gets misread as 255°C by the firmware. However, there are also some intermittent noise or ASIC reliability issues at low temperatures that 2.5.13 does not fix. I suggest that people try to avoid very low temperatures for another week or two while we look into those noise issues. Once an ASIC has an issue at low temperature, it makes the whole machine stop working until you isolate and disable that ASIC. While the ASICs usually recover after you bring the temperature back up, they don't always. This headache is really not worth the extra 30 or 50 GH/s you'd get from the lower temperature.