We have another feature article published by InfoQ's own Werner Schuster, where he elegantly explains a way to annotate object graphs with metadata to solve complex problems. An application where this type of problem may be faced is when a developer takes advantage of Ruby's ParseTree and needs to store specific type information about the nodes in the resulting tree for such things as source code parsers or static code analyzers.
Werner uses a combination of singleton classes and mixins to solve this problem and anyone looking for solution to a specific problem they are facing or what some knowledge for the future should go read Using Singleton Classes for Object Metadata now!