Yes, milk, and sugar. Eating sugar and milk causes weight gain in all patients, regardless of whether you are a gastric bypass or adjustable gastric band patient. These foods cause problems other than weight gain. In gastric bypass patients, eating sugar usually causes a variety of adverse symptoms called "dumping syndrome." "Dumping" occurs after eating milk, sugar or high-calorie liquids (even something as small as an M&M), causing you to feel heart palpitations, clammy, sleepy, shaky or gassy. Any combination of the above symptoms can occur either mildly or severely depending on what you ate. You just don't feel good.
As for other foods, patients undergoing either surgery will have certain food intolerances, but they vary from patient to patient. If you try something and it does not agree with you, leave it and try it again a few months later. Fruits, meats and fats should be avoided during the first three weeks after surgery.