This is usually caused when your vehicle is not used enough in cold temperatures, or when a system is not checked thoroughly during services. To prevent this from happening in your vehicle, you should use your air con during the winter months as this helps with the de-misting process and it also to keep the seals moist.
Pipe work and hoses can become loose causing fractures in the system, and hoses can rub together causing holes to form. Also, when pipe work or condensers corrode because of gritting salt during winter, leaks can occur. This can also happen when stone chips mix in the condenser.
Pressure switches, fuses, relays and cables can cause systems to stop running.