\documentclass{article}
\usepackage[showframe, nohead]{geometry}
\begin{document}
\vspace*{-3em}
\rule{3em}{3em}
\end{document}
Why does the rule protrude into the text area?
As a comparison, this is how it looks without the \vspace
:
And this is what I would have expected:
\vspace*{-4em}