Although all plants need water to grow, too much water can be deadly. When a plant is over watered, the roots are unable to breathe and will eventually rot. Over watering also leads to fungal diseases and mould. Once root rot begins the plant will begin to die but if it’s caught early enough, it can be saved.
A plant suffering from overwatering may appear to actually need water. The leaves will wilt and turn yellow, sometimes dropping off. Someone seeing this might believe that the plant needs water and add to the problem. But once again, feeling the soil is the best way to avoid this. When in doubt: Soil + dry = water. Soil + wet = don't water.
If a plant has been over watered take it out of is planter. If the roots still look healthy wrap the root ball in a layer of paper towels and leave them until the towels are saturated and repeat until the towels stay dry. Then re-plant. Don’t water it until the top two or three centimetres or so of soil is completely dry. If the roots look brown or mushy when the plant is taken from its planter it will need a little surgery. Start by rinsing off the soil off the roots and lightly pat dry, then examine them closely. Healthy roots are firm and generally white in colour. There are some exceptions. For example the roots of the Snake Plant are bright orange! If any of the roots are brown and mushy, take a sharp knife or pair of scissors and carefully cut them away. When complete, re-plant in fresh soil and water just enough to moisten the soil. Don’t soak it. If a large percentage of the roots were cut away, prune back the foliage to compensate.