During the Blitz and through the V-weapon attacks, Germany relied heavily on field agents to let it know what it was really hitting in England. If the agents consistently reported that attacks were striking outer North West London for example, German targets would be adjusted South East to compensate. Like when target shooting.

Except, those agents didn't actually work for the Germans. Twenty Committee (because twenty = XX in Roman Numerals, a Double Cross) had identified all the German agents and offered them either indefinite imprisonment for Espionage, or service as agents feeding bogus information to their German masters (and we can infer, the third alternative was death). You can guess what most of them chose.

Twenty Committee in effect ran German Espionage in WWII. If they had destroyed all these agents the Germans would have known and perhaps, in time, the Germans would have replaced them, but instead the Germans believed they had a working on-the-ground network of agents in Britain.

The point of the story is: "Just" having accurate information when actually somebody else controls your source of intelligence isn't actually having accurate information at all, it means you're a fool. Either you have control or you do not.