When the signature of a method of a base class and a method of a subclass that overrides it don't match, a call to the base class method may not be a valid call to the subclass method, and thus raise an exception if an instance of the subclass is passed instead. If following the Liskov Substitution Principle, in which an instance of a subclass should be usable in every context as though it were an instance of the base class, this behavior breaks the principle.