The handwriting on the postcard given to Gwenda in the kitchen is different to the postcard that Gwenda looks at on her dresser. Same words; the first one has messy handwriting, the second one is much more legible.
Outside the Savoy amongst the flags, is the flag of the EU which didn't exist until the early 1990's 40 years after Marple was set.
Outside the savoy hotel there is a taxi with a D registration. This came out in 1966 yet the film is set in 1930/40's.
A postcard supposedly from the missing Helen who disappeared in the 1930s has a stamp of Queen Elizabeth II, whose reign started in 1952.
The scene of the Savoy entrance in the 1950s the Hotel is flying the Maple Leaf Flag which Canada did not adopt until 1964.
During one of the flashbacks of the Funnybones performances, Edith plays a famous tune called "Sabre Dance" on the xylophone. However this would be impossible since the song was not composed until the 1940's and the flashbacks are said to be in 1934.
Julian Wadham plays Gwendas father, Kelvin Halliday, and curiously also Charles, Gwendas older admirer/fiance in India.