Confusion about the electrolyte in an electrochemical cell
As Ed said, that both copper and magnesium will reduce silver ions to elemental silver (you have to justify why in your answer. Hint: Look at the electrochemical series and compare the position of Mg, Cu, and Ag half cells. Another fundamental aspect is that one cannot apply Nernst equation if the metal dips in anything else besides its ions.

For instance, think little dip deeper, can we calculate the potential of copper electrode dipping NaCl solution (wrt to some reference cell?). In this case sodium ions will not react with copper or magnesium.