Often parents differ in their rules and expectations for their child. Mom might say, "You can't watch TV until your homework is finished"; but when she's away, Dad may say, "Go ahead and watch TV if you want to." Dad might insist that the child's bedtime is 8:30; Mom may say that stretching it until 9:00 is fine.
Similar conflicts can develop over issues like approaches to discipline or a child's choice of friends. When these inconsistencies occur, one parent inevitably undermines the authority of the other.