I was surprised to read that the Soviet Union learned of upgrades to the US Sidewinder air-to-air missile via an agent in Germany . . . and the German post office! War Is Boring reports:
Exploiting thick fog and careless guards, Manfred Ramminger – a KGB-agent in West Germany – entered Neuburg air base during the evening of Oct. 22, 1967. Together with his Polish driver Josef Linowski and German F-104 Starfighter pilot Wolf-Diethard Knoppe, he stole an operational AIM-9 from the local ammunition depot and transported it down the entire runway on a wheelbarrow to his Mercedes sedan, parked outside the base.
The 2.9-meter-long missile proved unwieldy. Ramminger broke the rear window and covered the protruding part with a carpet. In order not to attract attention of the police, he then marked the protrusion with a piece of red cloth, as required by law.
Reaching his home in Krefeld without any disturbance, Ramminger then patiently dismantled the Sidewinder. He kept the fuse for himself and personally handed it over to his KGB contact.
Finally, he packed all the pieces into a box and then brought it to the nearest post office, from where he shipped it – by air mail – directly to Moscow. In order to avoid any problems with the German or Soviet customs, Ramminger declared the content of the parcel as being for “low-grade export.”
Due to the weight of the parcel, the post charged him $79.25.
Air transportation services were making mistakes back then at least as often as they make them nowadays, and thus Ramminger’s parcel first traveled from Frankfurt via Paris to Copenhagen, then back to Düsseldorf, before finally reaching Moscow – 10 days late.
Ramminger and his aides were all arrested in late 1968 and jailed for four years. But by then the Soviets had already begun copying the next-generation Sidewinder.
There's more at the link.
The full article examines how the Soviet Union copied the original Sidewinder, then produced an improved version following the theft described above. It's a pretty amazing story. Recommended reading.