Combining two different types of magic is bad.
In a world of Functional Magic, there are many different types of magic and phlebotinum, each with its own complex set of rules and laws. And they each work pretty well as long as each one is kept separate. However, mixing magics is almost always disastrous. The two magics tend to interact in fairly unexpected ways, sometimes appearing far bigger or smaller than expected and sometimes creating an entirely new magic altogether. Needs More Examples, Needs a Better Description