Because if the plan is crazy, the enemy won’t have installed countermeasures.
Via Schneier, a bit of crazy-like-a-fox counterintelligence
Having lost many troops and civilians to bombings, the Brits decided they needed to determine who was making the bombs and where they were being manufactured. One bright fellow recommended they operate a laundry and when asked “what the hell he was talking about,” he explained the plan and it was incorporated — to much success.
The plan was simple: Build a laundry and staff it with locals and a few of their own. The laundry would then send out “color coded” special discount tickets, to the effect of “get two loads for the price of one,” etc. The color coding was matched to specific streets and thus when someone brought in their laundry, it was easy to determine the general location from which a city map was coded.
While the laundry was indeed being washed, pressed and dry cleaned, it had one additional cycle — every garment, sheet, glove, pair of pants, was first sent through an analyzer, located in the basement, that checked for bomb-making residue.
I am reminded of another story; I can’t recall if I read this somewhere or heard it as an Nth-hand retelling. Some spooks were trying to bug a building, which required drilling into a wall and depositing the eavesdropping device, but such an act would set off a motion/sound sensor. Someone came up with an idea: during every thunderstorm, when thunder was rattling the windows a bit, shoot mints at the windows of the targeted room and set the sensors off. Eventually, security would get tired of checking out the alarm and notice the correlation of thunder and alarm, and just shut the system down during a storm. Mints were used because they would quickly dissolve in the rain, in case anyone came around to check the possibility of an outside instigator. Once the spooks noticed that the sensors were no longer tripping and provoking a response, they planted the bug.
As cunning as a fox who has just been appointed Professor of Cunning at Oxford University.
My favorite example of this sort of thing:
During WWII, Jasper Maskelyne, the famous stage magician, was given the task of making the hulk of a WWI battleship sitting in a North African cove look like a modern, functional cruiser. This would cause the Germans to route supply convoys away from that stretch of coast. He had whatever resources he could scrounge up.
So what he did was to cover the WWI hulk with camouflage netting, with canvas panels attached to crudely disguise it as the WWI hulk. German reconnaisance aircraft, flying at high altitude, returned photographs of the result. German photo analysts, presumably, looked at the photos and sneered, “The British think we can’t tell the difference between a rusty hulk, and a cruiser with some camouflage and canvas over it! It’s not even a good fake! It’s an insult!” and routed the convoys away.