Using the scrlayer-scrpage package with a landscape environment from the pdflscape package creates a pair of overfull \hbox badboxes.
Some users have commented that they do not receive the badbox.
Version Information:
I'm using Tex Live 2014 (TeXworks, pdfLaTex).
*File List*
article.cls 2007/10/19 v1.4h Standard LaTeX document class
size10.clo 2007/10/19 v1.4h Standard LaTeX file (size option)
scrlayer-scrpage.sty 14/12/2013 v0.9.1548 package (end user interface for scrlayer)
scrlayer.sty 31/10/2013 v0.9.1480 package (defining layers and page styles)
scrkbase.sty 2013/12/19 v3.12 KOMA-Script package (KOMA-Script-dependent basics and keyval usage)
scrbase.sty 2013/12/19 v3.12 KOMA-Script package (KOMA-Script-independent basics and keyval usage)
keyval.sty 2014/05/08 v1.15 key=value parser (DPC)
scrlfile.sty 2013/12/19 v3.12 KOMA-Script package (loading files)
pdflscape.sty 2008/08/11 v0.10 Display of landscape pages in PDF (HO)
lscape.sty 2000/10/22 v3.01 Landscape Pages (DPC)
graphics.sty 2009/02/05 v1.0o Standard LaTeX Graphics (DPC,SPQR)
trig.sty 1999/03/16 v1.09 sin cos tan (DPC)
graphics.cfg 2010/04/23 v1.9 graphics configuration of TeX Live
pdftex.def 2011/05/27 v0.06d Graphics/color for pdfTeX
infwarerr.sty 2010/04/08 v1.3 Providing info/warning/error messages (HO)
ltxcmds.sty 2011/11/09 v1.22 LaTeX kernel commands for general use (HO)
ifpdf.sty 2011/01/30 v2.3 Provides the ifpdf switch (HO)
ifxetex.sty 2010/09/12 v0.6 Provides ifxetex conditional
supp-pdf.mkii
pdftexcmds.sty 2011/11/29 v0.20 Utility functions of pdfTeX for LuaTeX (HO)
ifluatex.sty 2010/03/01 v1.3 Provides the ifluatex switch (HO)
epstopdf-base.sty 2010/02/09 v2.5 Base part for package epstopdf
grfext.sty 2010/08/19 v1.1 Manage graphics extensions (HO)
kvdefinekeys.sty 2011/04/07 v1.3 Define keys (HO)
kvoptions.sty 2011/06/30 v3.11 Key value format for package options (HO)
kvsetkeys.sty 2012/04/25 v1.16 Key value parser (HO)
etexcmds.sty 2011/02/16 v1.5 Avoid name clashes with e-TeX commands (HO)
epstopdf-sys.cfg 2010/07/13 v1.3 Configuration of (r)epstopdf for TeX Live
MWE
\documentclass{article}
\listfiles
\usepackage{scrlayer-scrpage} % improved header commands. [supercedes `fancyhdr' package].
\usepackage{pdflscape} % landscape page layout.
\begin{document}
\begin{landscape}
a
\end{landscape}
\end{document}

\usepackage{mwe}andlipsum? They seem entirely superfluous. And you don't needgraphicxorfloatfor the MWE, either, although presumably those are not superfluous in your document itself. – cfr Sep 03 '15 at 23:20a. I reduced the code in the question, as prescribed. – kando Sep 04 '15 at 12:27mweandlipsumdoes nothing. Loading the first is unnecessary anyway and loading the second is pointless if you don't use it. To demonstrate what you said, you'd need to include a figure and/or text in the document. As I say, I get no bad boxes with justa. – cfr Sep 04 '15 at 13:272015/08/11 v3.18.2175…) – cgnieder Sep 04 '15 at 13:52