Chulin 97 - One-In-Sixty
If one neglected to remove the sciatic sinew and cooked the thigh together with the sinew in it, then if the thigh is small enough, so that the sinew imparts a flavor to it, the complete thigh is forbidden to be eaten. However, how do we differentiate between the two meaty tastes? We imagine that the sinew is made of meat, and the meat is turnip, and estimate whether this amount of meat will impart taste to the turnip.
If a sciatic sinew was cooked with other, permitted sinews, and afterward one recognized the sciatic sinew and removed it, then the whole mixture is forbidden only if the forbidden sinew imparts its flavor to it. Later Sages estimated to be the volume of one-in-sixty. However, if one does not recognize the prohibited sinew, then the entire mixture of forbidden, because any piece might be the forbidden one. In any case the soup is prohibited if there is forbidden taste in it. The same law holds true if a slice of non-kosher meat or non-kosher fish was cooked together with kosher slices.
Art: Jozef Israels - The Frugal Meal