There sure is! You will end up with a different amount of flour: when the recipe calls for "1 cup flour, sifted" measure the flour first and then sift. When your recipe calls for 1 cup sifted flour, it means you measure the sifted flour to 1 cup. Think of it this way: the comma divides two instructions (sifting and measuring). With no comma, you only have one instruction to follow: measuring. (You are measuring an ingredient called "sifted flour").