Star Trek VI: The Undiscovered Country - Uncredited Role
I don't know if it was intentional or not. Ethan Phillips had a role on ST: First Contact, as did Robert Picardo (both from ST: Voyager).
But Colonel West, the Starfleet officer who gave the briefing on Operation: Retrieve to the President was none other than Rene Auberjonois, Odo of ST: Deep Space Nine, and his role went uncredited.
This is not a slip-up. The original theatrical release (U.S.) did not include the portion of the scene in the Federation President's office where Col. West outlined his plan to rescue Kirk & McCoy. Also at the climax of the film, the bit where the Klingon assassin is revealed to be none other than Col. West was not included. Rene Auberjonois was uncredited because all of his scenes were cut out.