Factual error: A sign says "REMEMBER THE ENEMY IS LISTENING" in Chicago typeface, but this font wasn't invented until 1984. (01:15:50)
Factual error: When Sam reads the newspaper, one article claims that 254 people were killed in a Paris air raid. This episode takes place in February 1941, when Paris was already occupied by the Germans, and England did not bomb Paris. Oddly enough, this same article appears in a previous episode.
Answer: He doesn't actually wear it all the time even in summer. In some episodes in the summer he simply wears his suit with jacket.