@nony - I believe that it’s the job of the attorney to ferret out anything that the client may have which could impact the case one way or another; if it’s later found out that the attorney had evidence which could have affected the outcome of the case and didn’t volunteer it, that’s enough to cause the entire case to be retried. That’s my understanding away.
I’ve seen TV shows where someone was convicted of a crime and later found to be innocent because of exculpatory DNA evidence. It was then shown that the prosecution had this information all along but didn’t volunteer it. That’s a big no-no. I imagine you could be held in contempt of court for doing something like that.