Spoiled milk is basically lactic acid, which is formed due to the bacteria changing the milk sugar to acid. When milk is left in a warm place, certain bacteria in the milk start to multiply. They feed on the milk sugar and change it to lactic acid which turns the milk sour. It may taste disgusting, but this amount of lactic acid isn’t bad for the body.
In fact, this is the first process before milk is turned into yoghurt or cheese. It is also used in place of baking soda in baking. Hence, when you accidently drink some bad milk, it will not harm you. However, the bacteria present may result in some indigestion if taken continuously.
By 'spoiled milk' I think you refer to milk that has 'gone off' or is past the state where it can be called 'fresh'. Such milk that is old, or has been left in a warm place will be undergoing change. The bacteria feed on the lactose at a rapid rate causing it to convert into a mild lactic acid. This tastes sour and to curd. However unpleasant, this is not harmful for it forms the base of yoghurt and cheese.
This soured milk is useful in baking, as it can be a substitute for baking soda. In times gone by, the housewife depended on 'off' or soured milk for some of her baking recipes such as biscuits, flapjacks an sourdough. You need not worry, therefore, if you have drunk some spoiled milk the tasted sour and with a thickened texture like curd. It will not harm you.
The consequences of drinking spoiled milk can be mild. However, sometimes they can be quite nasty, such as a sick stomach. You are likely to throw it all up within an hour of drinking spoiled milk. However, people who drink spoiled milk are likely to suffer from food poisoning due to all the harmful and unhealthy bacteria that has grown inside the milk.
Sometimes the food poisoning won’t have a significant effect on one’s everyday life. In severe cases however, it can last for a few days. The effects also depend on when and where the milk has become spoiled.