Discovered a serious error in a reviewed paper after submitting the review, what to do? I have been reviewing a paper where the authors apply a computational method from an earlier paper they published to a specific system of general relevance. The paper seemed ok, and I recommended major revision in the review.

After submitting the review, I've been working with this computational method myself and discovered that the way the authors applied the method is seriously flawed, so that it makes in general no sense to interpret the result in any way without further tests. If I had noted this problem earlier, I would probably have recommended rejection in my review.

What's the recommended course of action in this situation? Should I notify the editor of this additional discovery, so that the authors can directly take care of it in their (presumably ongoing) revision, or is it better to just wait for a request to review the revised version?

I haven't heard any decision from the journal so far, but I don't think that the paper will be accepted without revisions.
What's the recommended course of action in this situation? Should I notify the editor of this additional discovery, so that the authors can directly take care of it in their (presumably ongoing) revision,


Yes, I think so.  This seems to be in the best interest of all involved: you, the authors and the journal.  It may be tempting to feel "embarrassed" about this or feel loathe to hold up the stately train of the editorial process, but you should resist these temptations: part of being a professional academic is being completely willing to change and adjust to new information and/or new insight you've acquired.


  or is it better to just wait for a request to review the revised version?


No, I don't think so.  Put yourself in the authors' shoes: wouldn't you like to have this information as soon as possible?  Revising a paper without knowing about a serious -- potentially fatal -- error sounds like it could be a waste of their valuable time.  Moreover, the longer they think that the paper will be accepted with revisions, the more disappointed they will be when they learn about the true situation.

I would write up carefully your description of the error as though it were part of the original referee report.  In effect this does become a new referee report, but you don't necessarily need to edit this into the old referee report: having already submitted that, the matter of it is to convey the new information.  Of course, the final decision about in what manner to inform the authors lies with the editor.