It is an error arise with package option \mhchemoptions{arrows=pgf}
after last update 2016-08-08
to v4.05
. Somebody can confirm?
\documentclass[]{article}
\RequirePackage[version=4]{mhchem}
\mhchemoptions{arrows=pgf}
\begin{document}
\ce{2H2 + O2 -> 2H2O}
\end{document}
\usepackage[version=4,arrows=pgf]{mhchem}
. I think this might be the intended behaviour, if I understand the manual correctly.