The attorney-client privilege is strongly protected so a client can tell the attorney the truth about what happened without fear of that information being used against him. However, there seems to be one key limit to this privileged, due to the fact that a attorney can not actively encourage or support perjury by, for example, encouraging his client to lie about an incriminating detail the client has already shared with the attorney.
What is a attorney suppose to do when his attorney-client privileged comes into conflict with the need to avoid supporting perjury? If the attorneys client starts making claims under oath that conflict with the, presumably truthful, depiction of events as described by the client to the attorney in private earlier? To make it clear I'm refering to cases where the attorney did not originally encourage or anticipate the difference until his client started to say something different on the stand.
I imagine to some degree this happens all the time. Human memory is imperfect and people can mis-remember something or make small mistakes in the recollection all the time. A client is likely also tempted to bend the truth slightly in their own defense, and if it's a small enough thing (like underselling how well they knew another accused individual) it's likely considered more ethical to let it go then risk violating privileged by calling the client out on such a dependency?
However, there is no doubt a point in which it becomes clear a client is veering into blatant misrepresentation to help their case; except that the attorney, who is the only one who has heard the full unbiased version from their client, may be the only one who is aware of this discrepancy. How is an attorney suppose to handle such a situation without violating attorney client privileged, or further harming his clients chances by explicitly stating he believes the client's testimony was dishonest; but also avoiding encouraging or becoming conspirator in perjury?