Journal was contacted by the sole author of an article that had been peer reviewed and accepted requesting the addition of a second author. The original author claimed that he had forgotten to include the co-author earlier. The journal is concerned about the risk that the new author has not done any work on the article and might get undue credit if their name was added.
COPE advice
The advice would be to put a hold on the publication of the article in question. The editor should then contact the original author for a full explanation of the circumstances and contributions of the "new" author. At the same time, the editor should contact the "new" author and ask for their perspective on the issue. Depending on the responses, the journal could either allow the addition, continue with the investigation (which may include asking the Institution for additional information) or disallow the addition. The editor could also suggest that an acknowledgement may be more appropriate than authorship.
Some journals require each author’s contribution be identified on submission, and this could be a process the journal could put in place so that this issue does not occur in the future. But even if it not the norm for the journal to ask about the contributions of the authors, the extraordinary circumstances dictate that the question should be asked: what did the second author contribute?