What are some of the causes of pinprick holes in evaporator coils in home central heat and air systems?

The chemicals in the freon can eat "worm hole" through the copper tubing in time. Other leaks may be around joints and fittings due to rust

Edit:

Make sure there's not an electrical grounding problem. Grounding electrical service to Plumbing used to be common, but it can increase the speed at which things corrode. The same thing could happen in AC.