A classic example is people who quit smoking, but end up gaining weight because they start overeating. This happened because the addiction was more than the nicotine of the cigarettes. It was the oral fixation. It was having something to hold on to, to raise to the mouth and wrap their lips around. Without the cigarettes, food is a common substitute to fulfill these unconscious desires.
What makes this interesting to me is the idea that bad habits are actually deeper than we think. The addictions we have come from a deeper need we are trying to satisfy. Stopping the bad habit doesn't fix the problem; it just creates a vacuum for some other bad habit to fill.
This makes for more interesting redemption stories, as well as making for good character development. People feel like they meet a character at their true core when they see the root of their unhappiness that has led to a self-destructive life. And it all starts from seeing them exchanging bad habits and realizing there is something deeper at at there.