When Daphne goes out to the gardens to get some air, in her panic, she removes the diamond necklace that was given to her by Prince Friedrich. When Simon turns up, she never retrieves said necklace, leaving it behind as they walk into another part of the garden. When she returns to the ball, she is still not wearing the necklace.
At the duel, the Doctor says "You think you have the skill to guide the path of a moving bullet?" It should have been "Ball" or "Musket Ball", the term "Bullet" began with cartridges and smokeless powder in the 1880s.
In the garden when Daphne removes the necklace from Prince Friedrich. It is seen on the terrace, where Cressida claims to have watched Simon and Daphne. No one ever notices its absence, including the prince though it is implied to be expensive.