An example I often link to is http://okmij.org/ftp/Computation/Subtyping http://okmij.org/ftp/Computation/Subtyping which uses a similarly simple example (bags

An example I often link to is http://okmij.org/ftp/Computation/Subtyping http://okmij.org/ftp/Computation/Subtyping which uses a similarly simple example (bags and sets) to show that supposedly "internal" changes can still break subclasses.