if it is sour, then it was caused by lactobacilli, in which you will be just fine. if you do get an upset stomach, its because of the low pH by the sour milk, causing a microbial upset of your intestinal flora and fauna.
however, ever once in a while, you will have milk that is decayed. this happens with pasteurized milk that has been contaminated by none lactobacilli microbes. most likely fungi.
it will taste like tooth decay. spit it out, its toxic and if you ingest too much you will die.
its foul tasting though if the milk is cold, you may not smell anything.
milk is a great food source so all sorts of microbes can live in there. pasteurization kills off all the good microbes in the milk, leaving a blank canvass for any microbe to colonize.
hope this helps