This happens when Sam and Carter are at Step's prior to them going into the dance together. Sam must return to the diner at twelve, so in order to do that Carter has her set an alarm on her cell phone. She hands him the phone and the sounds of buttons being pushed is heard, however, when the camera turns you can plainly see that the screen is black on the phone. It's kinda hard to set an alarm on a phone when the screen is black.