In areas like the Mid-Atlantic states where rock salt is used to control icing on the roads corrosion is a big problem for cars and trucks. This is an extreme example but it illustrates how corrosion can occur within wiring which is more common and usually undetectable. Corrosion here can cause a block heater to simply not work or cause a circuit breaker to trip. In theory it is also possible for fire to result.
You can help reduce the likeliness this type of failure by inspecting your block heater cord thoroughly especially at the plug end at the front bumper annually. Packing the plugs and lightly coating the electrical terminals at both ends of the cord with Motorcraft Silicone Brake Caliper Grease and Dielectric Compound (Part Number XG-3-A)or equivalent will help prevent water, salt and dirt from leeching into the wiring where corrosion can occur.