The episode is set in 1952. Characters are seen smoking filter-tipped cigarettes from hard card cartons. Filter tips were considered a speciality item until 1954, and only became widely popular from the 1960s. The hard carton with flip top was not introduced until 1955.
Mrs. Symmington can be seen quite clearly standing next to her husband attending the funeral for the murdered maid Agnes Brown. Mrs. Symmington was murdered and buried before Agnes Brown. In fact, Agnes attended Mrs. Symmington's funeral.
During the shots of the large initial cap letters being cut out for the poison pen letters, portions of lower case letters are included with the red capital letters. When the poison pen letters are shown later, there are only the red capital letters without any partial lower case.
After the headlamp has been mysteriously re-attached to the bike upon the accident, it is still lit. Even if the engine has stopped (No sound, rear wheel is not moving; the driver wouldn't be able to downshift that fast) and the battery has been thrown away.