Have you ever made a batch of cupcakes and found that you did not have enough batter to fill an entire 12-cup pan? Not having a full cupcake pan can result in uneven baking as the empty cups heat up faster than those which are filled with batter. To remedy this, either place an ice cube or 2-3 tablespoons water in each empty cup, then bake your cupcakes as directed. The ice/water-filled cups will heat up as quickly as the batter-filled cups, resulting in evenly baked cupcakes.
NOTE: This works for muffins too!