it seems that in the latest release of the showlabels package the command \showlabels{bibitem} breaks the output of bibliographic tags containing accented symbols, see MWE attached, and the difference commenting out \showlabels{bibitem} makes.
(From Michael Levitin, by email to Norman)
MWE:
\documentclass{article}
\usepackage{showlabels}
\showlabels{bibitem}
\usepackage[colorlinks,allcolors=blue]{hyperref}
\begin{document}
We will use \cite{Pol54}
\begin{thebibliography}{999999}
\bibitem[P{\'o}lya]{Pol54} G. P\'{o}lya, \emph{Mathematics and plausible reasoning}, in two volumes, Princeton University Press, Princeton, N. J., 1954.
\end{thebibliography}
\end{document}
I've been able to reproduce this, and I think I can see where the problem was introduced, between v1.8 and 1.9.
Fixed in revision 027a5da3c827, and released in v1.9.1