Lack of light, lack of fertilizer, or soil too dry. Unburned gasses in the air, spray or vapor damage from cleaning fluids, industrial pollution, or general pollutants in the atmosphere, possibly aphid or mite damage. Botrytis blight - fungal disease generally caused by dead leaves and spent flowers being left on the plant, too much humidity or poor ventilation. Crown, stem or root rot - caused by overwatering, especially during the winter months when plants are dormant and do not need much moisture. Fungi which usually develops when water is allowed to remain on the leaves.
Cold water can also be a cause of spotting.
Use room temperature water for misting and watering, and make sure the foliage dries before night. Powdery mildew is an airborne fungal disease. African violets and Begonias are particularly succeptible. Good cultural practices will eliminate many diseases and other houseplant problems. Many problems may be halted by removing damaged parts of the plant if they are detected early enough.