Please why does
\begin{document}
\begin{tikzpicture}
\foreach \x [remember=\x as \lastx (initially A)] in {B,...,H}{%
$\overrightarrow{\lastx\x}$,
}
\end{tikzpicture}
\end{document}
give
Missing character: There is no , in font nullfont!
in the log file while in document body? Removing the comma (,) after $ relieves the warning.

\listfilesbefore\begin{document}, and post the file versions here. Alternatively, update your packages (and I hope you are not on a 2009 release). – Peter Grill Nov 14 '12 at 20:13.logfile shows the lines aboutMissing characterquoted in the question. – Heiko Oberdiek Nov 14 '12 at 20:28