I remember testing a circuit to -40C in an environmental chamber and it seemed to work fine. I would put the unit in the chamber which I set to -40 and as it passed through every 10 degree step I would check it's performance. I thought everything was fine until I powered a unit down at -30C to change something and then powered it up. It didn't work. My units would run at -40C, but wouldn't start at -25C. This turned out to be a circuit design problem, not a component problem, but it is typical of the sort of problems you can get.