Once a rabbi who lived in a southern state called R’ Moshe Feinstein zt”l with a complicated problem.  R’ Moshe analyzed the matter and then said, “In a typical situation, we would follow the stringent opinion in this matter.  But in this case, where the woman is still childless after twelve years of marriage, it is correct to follow the lenient opinion.”

The caller was dumbfounded.  “But, Rosh Yeshiva!” he exclaimed.  “I did not mention this fact, nor did I mention the woman’s name.  How could the Rosh Yeshiva know that, indeed, she is childless after twelve years of marriage?”  “It slipped out of my mouth,” was all R’ Moshe could say.