Upset stomach. Bananas may cause abdominal cramps and or diarrhea. They also contain a lot of potassium so if you eat a lot you can increase your potassium levels which could be dangerous. Some people also report bananas may cause constipation.
Not much. Mostly, you gain weight. Too many bananas means you can satisfy hunger without getting a variety of foods needed for health. Bananas are good for carbohydrates, potassium and fiber, but shy on protein. Moderation in all things, even bananas.