Full log of the error with using knitr + changing OutDec options to ",". I filed the issue on https://github.com/yihui/knitr/issues/1081

error.log

processing file: teste.Rnw

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  ordinary text without R code


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label: unnamed-chunk-1 (with options) 
List of 1
 $ dev: chr "tikz"

Creating new TikZ metrics dictionary at:
	teste-tikzDictionary
Using TikZ metrics dictionary at:
	teste-tikzDictionary
\documentclass{article}

\nonstopmode

\usepackage{tikz}

\usepackage[active,tightpage,psfixbb]{preview}

\PreviewEnvironment{pgfpicture}

\setlength\PreviewBorder{0pt}

\newcommand{\SweaveOpts}[1]{}  % do not interfere with LaTeX
\newcommand{\SweaveInput}[1]{} % because they are not real TeX commands
\newcommand{\Sexpr}[1]{}       % will only be parsed by R


\usepackage{tikz}



\usepackage[T1]{fontenc}

\usetikzlibrary{calc}

\batchmode
\begin{document}
\begin{tikzpicture}
\node[inner sep=0pt, outer sep=0pt, scale=1,2] (TeX) { m};
\path let \p1 = ($(TeX.east) - (TeX.west)$),
    \n1 = {veclen(\x1,\y1)} in (TeX.east) -- (TeX.west)
    node{ \typeout{tikzTeXWidth=\n1} };
\makeatletter
\@@end
This is pdfTeX, Version 3.1415926-2.5-1.40.14 (TeX Live 2013/Debian) (format=pdflatex 2015.3.24)  26 JUL 2015 12:06
entering extended mode
 restricted \write18 enabled.
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**/tmp/RtmpdMc7CF/tikzStringWidthCalc.tex
(/tmp/RtmpdMc7CF/tikzStringWidthCalc.tex
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No file tikzStringWidthCalc.aux.
\openout1 = `tikzStringWidthCalc.aux'.

LaTeX Font Info:    Checking defaults for OML/cmm/m/it on input line 27.
LaTeX Font Info:    ... okay on input line 27.
LaTeX Font Info:    Checking defaults for T1/cmr/m/n on input line 27.
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LaTeX Font Info:    Checking defaults for OMS/cmsy/m/n on input line 27.
LaTeX Font Info:    ... okay on input line 27.
LaTeX Font Info:    Checking defaults for OMX/cmex/m/n on input line 27.
LaTeX Font Info:    ... okay on input line 27.
LaTeX Font Info:    Checking defaults for U/cmr/m/n on input line 27.
LaTeX Font Info:    ... okay on input line 27.
ABD: EveryShipout initializing macros (/usr/share/texlive/texmf-dist/tex/contex
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Package grfext Info: Graphics extension search list:
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(grfext)             \AppendGraphicsExtensions on input line 452.
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Preview: Fontsize 10pt
Preview: PDFoutput 1

! Package pgfkeys Error: I do not know the key '/tikz/2' and I am going to igno
re it. Perhaps you misspelled it.

See the pgfkeys package documentation for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.29 ...e[inner sep=0pt, outer sep=0pt, scale=1,2]
                                                   (TeX) { m}; 
Here is how much of TeX's memory you used:
 10089 strings out of 495004
 189311 string characters out of 6180662
 224771 words of memory out of 5000000
 13124 multiletter control sequences out of 15000+600000
 4403 words of font info for 15 fonts, out of 8000000 for 9000
 16 hyphenation exceptions out of 8191
 56i,2n,55p,425b,341s stack positions out of 5000i,500n,10000p,200000b,80000s

!  ==> Fatal error occurred, no output PDF file produced!
Quitting from lines 7-9 (teste.Rnw) 
Error in getMetricsFromLatex(TeXMetrics) : 
TeX was unable to calculate metrics for the following string
or character:

	m

Common reasons for failure include:
  * The string contains a character which is special to LaTeX unless
    escaped properly, such as % or $.
  * The string makes use of LaTeX commands provided by a package and
    the tikzDevice was not told to load the package.

The contents of the LaTeX log of the aborted run have been printed above,
it may contain additional details as to why the metric calculation failed.
Calls: knit ... print.recordedplot -> replayPlot -> <Anonymous> -> getMetricsFromLatex

Execution halted