I'm not sure of the source of this, although I recall at the time I first read it that it was reliable!
A senior pilot was tasked tp proceed to Salisbury Plain training area as an unexpected player [to the participants] in an Army exercise. [Well it would be wouldn't it!]. He was briefed to land at a specific grid reference and then 'phone a number for further orders. He duly arrived on site to find a large parade ground with a full dress inspection going on. After circling for a moment to confirm his position, he decided he must be part of the exercise and approached to land. It was not a pretty sight as the parade. minus caps scattered to the four winds.
Unperturbed, he despatched his crewman to make the phone call but, soon after, started to get somewhat anxious. The previous occupants of the parade ground were not taking too kindly to his presence, and several senior Army officers shook their fists at him. His colleague returned.
"What's the brief then?" asked the pilot.
"Well, put it this way Sir", replied his mate, "i've just tried to ring the grid reference".