The average stomach will hold about three-quarters of a gallon (that's 3 liters) of chewed food and beverages. You'll know when you've reached this limit by that post-feast queasy feeling that sets in. Some reports find that while the stomach will likely stretch to hold a little more than three-quarters of a gallon of food and liquid, it will actually spontaneously rupture when it's expected to accommodate as much as 1.3 gallons (5 liters) [source: Dahl].
If your stomach and intestines decide to close the gate on what you've eaten, that food and drink won't pass normally through to your bowel. Instead, it's coming back up, and sometimes forcefully. Vomiting is actually controlled by a "vomiting center" in the brain, and can be caused by a number of things, including food-borne illness (you can thank bacteria, viruses and parasites for most food poisoning), infections, some illnesses and pregnancy, as well as side effects of some medications or certain medical treatments (such as chemotherapy).
If you don't or can't vomit, your body will still deal with the unwanted stomach contents. Leaks may develop in the stomach walls, allowing partially digested food to seep into your body. (Suddenly, vomiting doesn't sound so bad.)