In general, when we create a FlyingCar, we ensure that we extend Car in a way that would work correctly. Hopefully we only rely on Car's interface that Car promises to keep unchanged; if we also rely on some other code in Car, we do it because we own the code.
On the other hand, this doesn't work in the opposite direction. When someone takes a FlyingCar tries to modify it to use it as a Car, there's no guarantee that things won't break, regardless of how careful the user is. After all, FlyingCar only promises that it will behave as a variant of Car if used as is; not that it will behave as Car after someone tries to takes some pieces out of it.
For example, FlyingCar might have modified the functions of various controls at construction. If its methods have been disabled, it won't become Car; it will be just broken.
