\documentstyle{article}
% The following definition changes the font that LaTeX 
% uses for the 'Large' font.  I have introduced a typo
% into the definition, ``\fontsiz'' should be ``\fontsize''.
%
% The  first time a \Large font is requested, an error will occur.
%
\renewcommand{\Large}{\fontsiz{17}{20pt}\selectfont}
\begin{document}

This text precedes the first section header.

% Note: LaTeX uses the \Large font in section
% headers...this will fail in a confusing way
% because the error is deep within the definition
% of \section where \Large is used...
%
\section{First Section}

This is the first and only sentence of the first section.

\end{document}