Always pack a black outfit when you travel, just in case.
This rule was instituted in 1952 after then-Princess Elizabeth learned the news of her father's passing while she and Prince Philip were on a royal tour of Kenya. Elizabeth hadn't brought any black dresses with her, so she stayed on the plane until one was delivered. Now, all members of the royal family are required to travel with funeral attire so they are prepared in the event of a sudden death.