There is a "design comprimise" in package lscape (in other packages it would be called "design flaw" ;-), of course). Environment landscape only rotates \textheight and \textwidth remains unchanged. However package geometry expects unchanged values, thus it gets surprised with \textheight having the old value of \textwidth.
The following example fixes this in package geometry's \Gm@vrule that draws the vertical lines. The fixed version uses \gmshow@textheight instead of \textheight. Outside environment landscape the macro \gmshow@textheight expands to \textheight. Inside it expands to \gmshow@@textheight that is initialized with the text height (available in \hsize) at the begin of environment landscape:
\documentclass[a4paper,twoside,11pt,openright]{report}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage[
outer=25mm,
inner=35mm,
vmargin=20mm,
includehead,
includefoot,
headheight=15pt,
showframe
]{geometry}
\usepackage{pdflscape}
\makeatletter
\newcommand*{\gmshow@textheight}{\textheight}
\newdimen\gmshow@@textheight
\g@addto@macro\landscape{%
\gmshow@@textheight=\hsize
\renewcommand*{\gmshow@textheight}{\gmshow@@textheight}%
}
\def\Gm@vrule{%
\vrule width 0.2pt height\gmshow@textheight depth\z@
}%
\makeatother
\begin{document}
\noindent
Hello. First page. Everything ok.
\begin{landscape}
\Huge
\noindent
top left\hfill top right\par
\vfill
\noindent
bottom left\hfill bottom right
\end{landscape}
\end{document}

landscape(so that is adds a marker on landscape pages) and geometry to get a correct result. – Ulrike Fischer May 24 '13 at 10:13geometrybe "aware" of thelandscapeenvironment? – Mario S. E. May 24 '13 at 11:22\textwidthdoesn't change)) It basically just applies\rotatebox{90}{}to the content of each page without changing the page geometry. – David Carlisle May 24 '13 at 11:39\textheightchange but not\textwidth? I would expect, based on your description that neither would change. – cfr Apr 19 '14 at 00:31\textheightwould also remain unchanged. – cfr Apr 19 '14 at 23:21\textheightand\textwidthare constant throughout the document so that I can save their values to macros at the beginning and count on the saved values for laying out every page, whether in portrait or landscape. If\textheightdidn't change in landscape, I could use that value which would be sensitive to changes. Or if\textheightand\textwidthwere switched, I could use them appropriately in landscape. The problem is that I can't access the\textheightat all in landscape as far as I can tell. – cfr Apr 20 '14 at 00:08\linewidthand\columnwidthare both set to the old value of\textheight– David Carlisle Apr 20 '14 at 00:29