If the mirror fogs, condensation will have formed on other parts as well. I had thought fogging was no more than a temporary inconvenience. However I was surprised to see this recent thread that indicates it can have a potentially calamitous effect:
The lesson is that when entering a warm area, such as a building, after the camera has been exposed to low temperatures, place the camera in a sealed bag and leave it there until its temperature has equalised with the surroundings. It also applies if the camera is taken from a cool air-conditioned area into warm, humid conditions. It also seems that the same problem can occur outdoors in exceptional circumstances.