The Standing Committee on Ethics and Professional Responsibility of the American Bar Association recently issued Formal Opinion 481, called “A Lawyer’s Duty to Inform a Current or Former Client of the Lawyer’s Material Error” in which it concludes that attorneys have a duty to disclose material errors in representation to their clients. You can read the opinion here.
Evidently, the duty is limited by "materiality," but what is it that makes an error "material." According to the opinion, a material error is defined as one that a “disinterested lawyer” would conclude is either “(a) reasonably likely to harm or prejudice a client, or (b) of such a nature that it would reasonably cause a client to consider terminating the representation even in the absence of harm or prejudice.”
Interestingly, however, the opinion finds that there is no duty to inform a former client if the lawyer discovers after the attorney-client relationship has ended that the lawyer made a material error in the former client’s representation.
You can find short comments on the opinion over at The Law for Lawyers Today and Professional Liability Matters.