Some LaTeX tutorials and at least one wellknown online reference manual explain the declaration of an appendix by an environment, they recommend to write: \begin{appendix} … \end{appendix}. Though this will be compiled without error it actually is not functioning like we expect of an environment. Everything that follows \end{appendix} will also be treated as part of the appendix!
The correct usage is:

\appendix

This command is defined by the standard classes by \newcommand, not by \newenvironment, there’s no \endappendix. For example here’s the original code of book.cls:

\newcommand\appendix{\par
  \setcounter{chapter}{0}%
  \setcounter{section}{0}%
  \gdef\@chapapp{\appendixname}%
  \gdef\thechapter{\@Alph\c@chapter}}

If you want to end the appendix and add further chapters or sections like list of figures etc. you would have to undo the changes made by \appendix or use just a common chapter or section labeled as appendix.

The appendix package provides more facilities for typesetting appendices and even allows subappendices.

This topic was discussed in the LaTeX Community Forum and on Matheplanet.