Here is a story from the Pennsylvania Disciplinary Board (via the Legal Profession blog).
An attorney was retained to pursue claims for injuries sustained by a passenger in a plane that collided with a truck on the runway. In the suit, he asserted that the client's injuries were solely caused by the accident. The client so testified at her deposition. The client also testified that she had had no contact with the attorney prior to the plane accident.
However, this was false. The client had been in a car accident two months before and had retained the attorney to pursue the claim.
The attorney failed to correct her false deposition testimony, contending that he did not "contemporaneously realize" that the testimony was false.
I am sorry, what? So, at the time of the deposition the attorney had forgotten that the client was his client in another on going case? Or was it that the attorney did not realize the client lied when she said she had not contacted the attorney? Since he must have known she had contacted him, I am not sure it is difficult to see the statement was incorrect.
What sanctions would you impose? The Board imposed a public censure. The opinion is available here.