On the Value of Empirical Research
To adjust and correct settings in the V-1 guidance system, the Germans needed to know where the V-1s were landing. The V-1 operators asked German intelligence to get this impact data from agents in Britain. All these agents were double agents under British control (the Double Cross System), so the British sent deceptive data to the Germans.
V-1 impacts were often reported in the public press, so the British did not send false reports. Instead, they sent locations of actual impacts, especially those in central London, or "long" (to the northwest), but they omitted most of the impacts falling "short" (to the southeast). The Germans were thus induced to adjust their range settings down, shifting the "Mean Point of Impact" (MPI) away from central London to less populated areas.
[..]
This deception operation suffered some interference from a German double agent in Portugal codenamed Arabel, who fraudulently claimed to have agents in Britain. He sent in a lot of fake impact reports which the Germans accepted. Fortunately, thanks to Ultra, the Allies read his messages and were able to adjust for them.
http://en.wikipedia.org/wiki/V-1_flying_bomb#Operation_and_effectiveness
V-1 impacts were often reported in the public press, so the British did not send false reports. Instead, they sent locations of actual impacts, especially those in central London, or "long" (to the northwest), but they omitted most of the impacts falling "short" (to the southeast). The Germans were thus induced to adjust their range settings down, shifting the "Mean Point of Impact" (MPI) away from central London to less populated areas.
[..]
This deception operation suffered some interference from a German double agent in Portugal codenamed Arabel, who fraudulently claimed to have agents in Britain. He sent in a lot of fake impact reports which the Germans accepted. Fortunately, thanks to Ultra, the Allies read his messages and were able to adjust for them.
http://en.wikipedia.org/wiki/V-1_flying_bomb#Operation_and_effectiveness