From a New York Times story on the botched Stevens trial:

For example, a witness for both the government and defense, Rocky Williams, was sent home to Alaska by prosecutors who did not tell defense lawyers, an act that angered Judge Sullivan. Ms. Morris said the decision was made because Mr. Williams was gravely ill, not because prosecutors, after interviewing him, had decided he might help the defense case.

But Mr. Joy said a prosecutor, Nicholas Marsh, concocted the scheme to send Mr. Williams away after prosecutors held a mock cross-examination in which he did not perform well.

Still, there is considerable evidence that Mr. Williams was truly sick, including the fact that he has since died.