Actually, the bombs were already falling short of the target. The British instructed captured German spies to report they were hitting their targets. They were quite subtle about how they did it -- they would simply omit news about the bombs that fell short.

My favourite story is about Battle of the Beams (http://en.wikipedia.org/wiki/Battle_of_the_Beams), where German bombers were guided by radio navigation. The British started broadcasting their own radio signals to make it appear to the German planes that the beam was slightly bent. It meant they could get the bombers to drop their bombs in the middle of nowhere.