I encounter this problem with both the MSCS style file, which is not a standard LaTeX package but an in-house style of Cambridge Journals, and also with the ACM sigplanconf style. I'm not sure if it's appropriate to ask about in-house styles here, but if so I would appreciate some help. I don't believe this is related.
I get a spurious period at the end of bibliography items whenever they contain DOIs, as shown below (the very last period should not be present):

Here's the code for the above; you'll need to put mscs.cls in the same folder.
\documentclass{mscs}
\usepackage[authoryear]{natbib}
\begin{filecontents*}{bib.bib}
@article{milner92,
author = {Robin Milner and Joachim Parrow and David Walker},
title = {A Calculus of Mobile Processes, {I} and {II}},
journal = {Inf. Comput.},
volume = {100},
number = {1},
pages = {1--77},
year = {1992},
doi = {10.1016/0890-5401(92)90009-5},
}
\end{filecontents*}
\begin{document}
\citep{milner92}
\bibliographystyle{plainnat}
\bibliography{bib.bib}
\end{document}
Note: natbib doesn't seem to be essential to the problem, but I would like a solution that works with it.
