Overwatering a houseplant prevents proper drainage, which can in time lead to root rot, a malady that causes the roots to turn mushy and brownish black from a lack of oxygen. Signs of root rot include a plant that is slowly wilting and leaves that are turning yellow and falling off. To save a plant from root rot, try removing it from the soil and washing the roots clean. Then, cut the roots back to remove the diseased tissue, and repot the plant in fresh soil after cleaning and disinfecting the container.
flickr.com via David B. Gleason