While it's not considered to be good practice to eat foods that have passed their expiration dates, some foods are still considered edible if they are a few days past the date. While yogurt is technically 'spoiled' already, it does still have the ability to go bad. Eating expired yogurt could put you at risk for developing diarrhea, upset stomach or even food poisoning. The growth of bad bacteria is often the leading cause of sickness due to food consumption.
In most cases, nothing happens if you eat expired yogurt. Unopened yogurt lasts much longer than the expiration date, as does opened yogurt. As long as it smells good, and the taste is normal, you should be fine. If you become ill, please seek medical attention. Remember, if in doubt, throw it out.