Q:

Breaking (\par) environment inside of tikzpicture

I'd like to add a transparent box over an image and add some text, three names, one below the other. The approach I'm following give me the transparent box the way I want, but I'm not able to get the names one below the other. What am I doing wrong?  
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}

\begin{document}
  \begin{tikzpicture}

\begin{tikzpicture}
    \node[preaction={fill=white,fill, opacity=.6},rounded
        corners=1ex,font=\fontsize{16pt}{16pt}\scshape] 
        {{Some text A} \par {Some text B} \par {Some text C}\par};
  \end{tikzpicture}

\end{document}

A:

You have two \begin{tikzpicture} in your document. ;-)
Leaving that aside, if you wish to force linebreaks inside a TikZ node, one of the ways to do this is to use the align option in the node (described in 17.4.3 in pgfman.pdf, p.223ff) and force linebreaks via \\ (there may be other ways, but that's the approach with which I am familiar).
Does this produce the desired result?
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}

\begin{document}
\begin{tikzpicture}

    \node[
        preaction={fill=white,
        fill,
        opacity=.6},
        rounded corners=1ex,
        align=left,
        font=\fontsize{16pt}{16pt}\scshape
    ] 
        {Some text A \\ Some text B \\ Some text C};
\end{tikzpicture}

\end{document}

There are other options for align besides left (although left is probably what you'd want here):

flush left
right
flush right
center
flush center
justify

Also of interest might be the text width option (not necessarily for this example, just in general for handling text in TikZ nodes). It's described in the same section of the PGF manual. For explanations of the options and a visual demonstration of their effects, a look at the manual is probably the easiest way.
The PGF manual can be found here: http://mirrors.ctan.org/graphics/pgf/base/doc/pgfmanual.pdf

