A thebibliography environment that's been built by hand may well contain citations of things for which there is no reference in the document. This package produces a warning for each such thing. (Note that the package is of no use for users of bibtex, since bibtex doesn't create citations unless there is a reference.) This package was implemented by C. V. Radhakrishnan and C. V. Rajagopal. See also bibtex.