"You shall not cook a goat in its mother's milk" — the Mekhilta derives from this verse that the cooking prohibition applies specifically to meat and milk, and not to other combinations of forbidden foods.
One might construct a logical argument: if meat and milk — each of which is individually permitted for consumption — may not be cooked together, then surely two foods that are individually forbidden should also be prohibited from being cooked together. The a fortiori seems airtight. If permitted items create a prohibition when combined, forbidden items should certainly create a prohibition when combined.
The Torah preempts this logic by specifying: "You shall not cook a kid in its mother's milk." The specificity is deliberate. It is only meat and milk that are singled out for this cooking prohibition. Other forbidden combinations — mixing two types of non-kosher food, for example — are not subject to an additional cooking prohibition.
This ruling demonstrates an important principle in Jewish law: a fortiori arguments cannot create new prohibitions that the Torah chose not to state. The Torah was aware of the logical extension and deliberately refused to make it. The specificity of "a kid in its mother's milk" is a boundary marker. It tells us exactly how far this prohibition reaches and where it stops. What the Torah chose to prohibit, it prohibited. What it chose not to extend, no amount of human logic can extend.