“Argument of @sect has an extra }.”

If you get this error, it means that you have put what’s called a fragile command in a thing called a moving argument. In English, that means you’ve put a \citeasnoun command (from the harvard package), a footnote command or similar in something like a section heading, like so:

\documentclass[a4paper,12pt]{article}
\usepackage{harvard}

\begin{document}

\section{Discussion of \citeasnoun{bloggs:2011}}

\citeasnoun{bloggs:2011} argues that \dots

\begin{thebibliography}{1}
\harvarditem{Bloggs}{2011}{bloggs:2011}
Bloggs, J.  \harvardyearleft 2011\harvardyearright, Publisher, UK.
\end{thebibliography}

\end{document}

This will give you the error:

Argument of \@sect has an extra }.

To fix this, all you have to do is \protect the fragile command in the section as follows:

\documentclass[a4paper,12pt]{article}
\usepackage{harvard}

\begin{document}

\section{Discussion of \protect\citeasnoun{bloggs:2011}}

\citeasnoun{bloggs:2011} argues that \dots

\begin{thebibliography}{1}
\harvarditem{Bloggs}{2011}{bloggs:2011}
Bloggs, J.  \harvardyearleft 2011\harvardyearright, Publisher, UK.
\end{thebibliography}

\end{document}