Hydroelectric dams are a good example of this. At first sight they seem to be all to the good as far as reducing GHG goes. However, not only does their construction often mean the loss of much land,and the displacement of its animals and humans, it can also end it up to be quite a big GHG emitter. Much of the organic matter washed into the lake behind the dam decays anaerobically in the depths, rather than aerobically as it would have done in the original river. This anaerobic break down produces lots of methane, a more powerful greenhouse gas than the normal CO2.