Recent news stories reveal that Israel, in collaboration with Greece, conducted training against the deadly Russian air-defense system, the S-300, which is now deployed in Syria and is also being sold to Iran. That Israel and Greece [and ultimately their master - the US] are able to sort of reverse-engineer the working of the S-300 system shines spotlight on the risks inherent in foreign sales of critical military gear. As part of the Cyprus Missile Crisis, the S-300 fell into the hands of a NATO member, and it is certain that America must've pounced upon the system to understand its inner workings, to copy its superior characteristics and to train against it.
Of course, there's a risk for Israel, Greece and the US too. Assuming that Russia keeps updating the software of the S-300, it's possible that Western nations train against an older firmware version of the S-300, and are taken aback in the battlefield when a system with the latest software version is able to lock-on despite the evasive measures devised by the West.