Something I have done in a similar situation is to write about the content of the paper on the nLab wiki, including the correction. I think this is a great solution, because putting the material on a wiki is itself a service to the community and to the original author (increasing the potential audience for his/her work and helping to disseminate it), and I think including the correction when you do so is unobjectionable, since the culture of a wiki (anyone and everyone is expected to add to it, improve it, and fix mistakes) is different from the culture of publishing refereed papers, or even arXiv preprints.

Of course, this may not be an option when working in a field where there doesn't yet exist any wiki comparable to the nLab -- but then clearly the solution is to start one! (-:O