I used to write my papers on Windows, with all my citations in a .bib file, which is imported like this:
% Bibliography
\bibliographystyle{plain}
\bibliography{MyBibliography}
This all compiled fine. I recently switched to Mac (using TexShop), which produces nearly the same, except that in front of every citation in the text, it writes '(author?)'. Like this:

Other than that, the references, the numbers, and the bibliography itself are exactly the same. This is very irritating. What could have gone wrong here?
\citeand the key that is given in thebib-database. If that's not it, please provide a minimal working example. – Johannes_B Jun 14 '15 at 11:27