This is pdfTeX, Version 3.14159265-2.6-1.40.15 (TeX Live 2014) (preloaded format=pdflatex 2014.11.3)  22 SEP 2016 13:28
entering extended mode
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**runaway_argument.tex
(/compile/runaway_argument.tex
LaTeX2e <2014/05/01>
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Runaway argument?
{Introduction \^^M\end {document} 
! File ended while scanning use of \@xdblarg.
<inserted text> 
                \par 
<*> runaway_argument.tex
                        
I suspect you have forgotten a `}', causing me
to read past where you wanted me to stop.
I'll try to recover; but if the error is serious,
you'd better type `E' or `X' now and fix your file.

! Emergency stop.
<*> runaway_argument.tex
                        
*** (job aborted, no legal \end found)

 
Here is how much of TeX's memory you used:
 221 strings out of 494521
 2433 string characters out of 6168058
 48172 words of memory out of 5000000
 3561 multiletter control sequences out of 15000+600000
 6675 words of font info for 24 fonts, out of 8000000 for 9000
 319 hyphenation exceptions out of 8191
 23i,6n,17p,124b,187s stack positions out of 5000i,500n,10000p,200000b,80000s
!  ==> Fatal error occurred, no output PDF file produced!