When I was a student at the Beth Jacob high school in Mtl, our principal,
Rabbi Shneur Aisenstark, once spoke about Rabbi Feinstein's heter
(allowance) to drink milk that was not cholov yisroel if it was government
supervised. HOWEVER, our principal then added these words that made on me
the greatest impression: When Rabbi Feinstein was visiting at one of his
relative's homes who was not machmir (stringent) in cholov yisroel, he
mistakenly drank from the milk that was NOT cholov Yisroel. When his
relative told him this, Rabbi Moshe forced himself to vomit all the
contents of the milk that he had been drinking. Need I say more?
Maayan, Israel