circle-ellipse problem

The problematic situation where a base class contains methods which mutate an object in a manner which may invalidate a (stronger) invariant found in a derived class, violating the Liskov substitution principle. For example, if a circle is modelled as a subtype of an ellipse, and the ellipse has a method that stretches it on one axis, the circle must also implement that method despite it making no sense for a circle.