The US and NATO had instances of overestimating the USSR and Eastern Bloc's fighter capabilities.

From this recently (2008) declassified Air Force document on the development of the F-15 Eagle air superiority fighter, numerous references are made to the new MiG-25 Foxbat.

If you look on page 25 of the document, it mentions:

Secondly, the Soviets had displayed several new fighters at the July 1967 Moscow Air Show, and one of these - the Foxbat - was regarded as superior to existing and projected American counterparts (including the "Representative F-X" as described in the August 1967 CFP) in speed, ceiling, and endurance.

There are other references to the Foxbat in this document, all citing the fact that the MiG-25's appearance - with its big engine and large wings giving it the appearance of being a high speed and highly maneuverable platform that was superior to the initial F-X program requirements.

In turn, the US updated its F-X program requirements which resulted in the F-15, a no-compromises air superiority fighter with big powerful engines, high maneuverability, and the latest radar systems.

It turned out, of course, that the MiG-25 was not an air superiority fighter. Rather, it was a high speed, high altitude interceptor with limited dogfighting capabilities. The large wings on the aircraft were required for its heavy weight, and not for maneuverability, as the US previously thought. These capabilities weren't fully revealed until Viktor Belekno defected with a MiG-25 in 1976.

So one could say that the US effectively created one of the world's great air superiority fighters rather inadvertently by overestimating what the Soviet Union had made with the MiG-25, the aircraft they thought would be replacing the MiG-21 and MiG-23.

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On a side note, perhaps a user of /r/AskHistorians can recall/dig up the thread but a similar question was once posed about Soviet Air Force pilot training. Retired Major General Williams (who's account on /r/ is now deleted) commented about the state of Soviet Air Force/Eastern bloc pilot training and what was learned after the Cold War ended - he stated that they were shocked to find how inferior the training was and the number of hours provided to pilots per year. He was a primary source to all that, of course.