rob: It’s T’shuvas HaRama Siman 104 or 105, if I recall correctly.
And mdd, truthsharer gave a perfect example of what I was referring to. R’ Moshe’s Shittah on Chalav Yisrael is probably a similar example. (Though I would word it slightly differently. I would say that Poskim think, “It’s not possible that so many Jews are drinking Trief; therefore I will make sure to prove that it’s Muttar.”