Q: Whom should I inform if I have spotted an error in a published paper?

First of all, you need to be absolutely sure that there is an error before you take any action. I would suggest that you go through the paper once again and also ask some senior colleagues to verify whether the error really exists. Once they confirm the error, it would be best to write to the corresponding author of the paper, informing him very politely that there seems to be an error in the paper. Possibly, the error could be the result of a misprint. It could also be that the authors had intended to cite one particular work, but have by mistake cited some other work by the same author. Mistakes like this do happen, and authors generally respond quite positively if someone points them out.

In all likelihood, you will receive a response from the corresponding author. However, in case you do not get any response within a reasonable time (about a couple of weeks to a month), you could write to the editor of the journal.