documentclass[a4paper, 12pt]{report}
usepackage[a4paper,left=4cm,right=2.5cm,top=3.5cm,bottom=3cm]{geometry}

begin{document}
begin{titlepage}
centering
today
end{titlepage}
tableofcontents
chapter{Chapter title}
section{Section title}
subsection{Subsection containing extremely lengthy heading obnoxiously} % toc overrun by this title, words picked deliberately to exhibit the flaw
end{document}

subsection[shorter title]{longer title}