The journal says they won't publish any more errata for missing citations:
An economics journal has corrected a paper for the second time for failing to cite previous studies — and said in a separate note that it no longer plans to publish similar errata, with rare exceptions.
In September 2015, we reported on the first erratum for “Incentives for Creativity” — a paper that analyzed ways of inspiring creativity in the workplace — after it failed to cite relevant papers. One year on, the same paper has another erratum for a similar reason: not citing relevant papers from another field. ...
In “Editors’ note regarding citations of other work,” the editors go on to say:
Those authors who feel that they should have been cited or that were cited inaccurately in an article that we have published will have to use other means, such as posting notices on their own websites or contacting key researchers doing related work directly, to notify the research community of their concerns.
The first erratum contains four literature review paragraphs with reference to four unpublished papers (cited as "mimeo"). The erratum authors say that the uncited authors had not complained about the missing citations.
I'm not sure if I would feel an obligation to cite an unpublished paper (in fact, many of these contain "not for citation" on the cover page because they haven't gone through peer review). Two of the four papers can't be found via Google Scholar except in the erratum (here and here). On the other hand, if I get an idea from an unpublished paper that I'm familiar with via a conference presentation or if I'd read the working paper I'd be more than happy to cite it.
The second erratum is because the published paper shares 31 words in instructions from another (uncited) experimental paper. In this case, the uncited authors did complain.
