The two main actors in this episode, John Qualen and Lorne Greene, died one day apart on 11-12 Sept 1987, over three decades after this episode was made.
In the opening title credit, Lorne Greene's last name is spelled Green; however, it's spelled correctly in the cast list in the closing credits.
The story from Stanley Ellin was already filmed in 1949 in TV series Suspense (1949), with the same title.
While Mr. X and Crabtree are talking, a calendar is on the wall in the background, turned to April. The episode aired in 1956 but the most recent April where the days lined up as shown on the calendar was 1953.