\documentclass{article}
\begin{document}

\section{Intro}

Here we start an article that will execute a huge number of undefined macros.

\section{Last heading here}
% Any nonsence that generates errors would do,
% let's just call \the on undefined macros in a loop:
\makeatletter%
\def\letters{Fuu}%
\def\letterloop{\edef\thisname{\csname\letters\endcsname} \the\thisname \g@addto@macro\letters{Fuu} \letterloop}
\letterloop

\section{This should never be entered}

or this.

\end{document}