What are the worst consequences of heart bypass?

Death, Heart Failure. The worst consequence of all would be death, but others would include heart failure (a weak heart), infection (mediastinitis), pericardial effusion (fluid around the heart), stroke, heart attack and others. Fortunately, in good hands these are pretty rare.
Complications. From the surgery itself including possibility of death. Let's focus on the benefits which can be substantial...Including increased life expectancy.