In correct Russian spelling convention, the Countess' name would be spelled Rossakov. Additionally, the name of a female of the family would end with "a", thus "Rossakova."
The "contralto" is actually a soprano.
Despite what they portray in movies and television acid does not work forever, specifically the miniscule amount of acid put into the lock would not dissolve enough of the pins to allow the lock to be opened. In addition, it would not work that quickly.
Despite being set in the 1930s, a gloved hand is holding a very modern looking Maglite flashlight scanning the room.
The trail from a modern jet aeroplane is visible on an exterior shot of Whitehaven Mansions.
When Poirot appears distracted by the Countess, Captain Hastings and Miss Lemon decide to conduct their own investigation. When they return to Poirot's flat they are shown coming through the door with keys on a chain hanging from the outside of the door lock. They close the door behind them forgetting the keys.