To change the orientation of a beamerposter the following usual code
\documentclass{beamer}
\usepackage[size=a4]{beamerposter}
\usepackage[landscape]{geometry}
\begin{document}
hey
\end{document}
will give the error:
! LaTeX Error: Option clash for package geometry.
Therefore I am forced to pass the options to the geometry package in the beginning with \PassOptionsToPackage{landscape}{geometry}
\PassOptionsToPackage{landscape}{geometry}
\documentclass{beamer}
\usepackage[size=a4]{beamerposter}
\begin{document}
hey
\end{document}
(see, for example, option clash for package xcolor)
However, the result is completely opposite. When the portrait option is provided the page is landscape and vice versa.
This can indeed be a source of confusion and misunderstanding! How to fix this problem?