Q: While my roommate was frying chicken, I accidentally spilled milk into the frying pan, rendering the chicken and frying pan non-kosher. What am I liable for?

A: Damage that is not physically discernible but only halachic is called hezek she’eino nikar. A person is legally liable for such damage only if done intentionally, but not if it is accidental (C.M. 385:1). This is because, in principle, damage that is not physically evident is not considered damage. Nonetheless, the Sages declared a person who intentionally damaged in such a manner liable - having to pay a fine - to prevent people from doing so. [Some authorities limit the liability to three specific cases mentioned in the Mishnah (Gittin 52b; see Shach 385:1.)]

Here, the taste of the milk is noticeable in the chicken. It is therefore considered hezek nikar, evident damage, and you are liable for the cost of the chicken (Ksav Sofer, C.M. #26).

However, there is no physically evident damage to the frying pan. Therefore, since it is hezek she’eino nikar, you are not legally liable if you spilled the milk by mistake (see Pischei Teshuvah, C.M. 385:1). Nonetheless, there may be a moral responsibility to pay (Pischei Choshen, Nezikin 1:[54]).