Removing sidebar from a single beamer frame

\documentclass{beamer}
\usetheme{Goettingen}
\usepackage{lipsum}
\usecolortheme{seahorse}

\begin{document}

\begin{frame}{title}

\lipsum

\end{frame}

\begin{frame}[plain]{title}
\advance\textwidth2cm
\hsize\textwidth
\columnwidth\textwidth
\lipsum

\end{frame}

\end{document}

Based on David's answer, you can do the following. I also suggest you to remove the navigation symbols because with this approach there is a problem in the specialframe.

\documentclass[c]{beamer}
\usetheme{Goettingen}
\usepackage{lipsum}
\usecolortheme{seahorse}
\setbeamertemplate{navigation symbols}{} % removes navigation symbols

\newenvironment{specialframe}
{
    \begingroup
    \advance\textwidth2cm % see beamerthemeGoettingen.sty for the number
    \hsize\textwidth
    \columnwidth\textwidth
    \begin{frame}[plain]
}
{
    \end{frame}
    \endgroup
}

\begin{document}

\begin{specialframe}
\frametitle{This is a special frame}

\lipsum[1]

\end{specialframe}

\begin{frame}{No special frame}

\lipsum[2]

\end{frame}

\end{document}