Consider the LaTeX code:
\documentclass[aps,pre]{revtex4}
\usepackage[normalem]{ulem}
\begin{document}
\sout{
asdas~\cite{a1989, b2001}.
}
\bibliographystyle{apsrev4-1}
\bibliography{references.bib}
\end{document}
where references.bib is just a .bib-file with some random entries with a1989 and b2001 keys.
I compile using
- Latex
- BibTex
- Latex
- Latex
At step 4, I get the following error
! Missing number, treated as zero
If I continue, it just ignores and does what it is supposed to.
This only happens when there is more than 1 citation on the same \cite. E.g. if I use
asdas~\cite{a1989}~cite{b2001}
it works fine.

latexdiffprovides otherways to mark up differences, e.g. via colour. May be you should investigate that. – Andrew Swann Feb 14 '13 at 17:37