Are there notable incident(s) where Russia/SU was wrongfully accused by Western powers in the 20th century?
I can think of two. When Gary Power's U2 spy plane was shot down over Russia initially Eisenhower falsely claimed the plane was lost civilian plane and denied it was on a spy mission. Eisenhower had to backtrack when Power's turned upon on Soviet TV. Ultimately Eisenhower was forced to admit the US had been flying spy missions over Russia for years prior to Power's flight being shot down (May 1960) by the Soviets.
U-2 SPY INCIDENT
Confronted with the evidence of his nation’s espionage, President Dwight D. Eisenhower (1890-1969) was forced to admit to the Soviets that the U.S. Central Intelligence Agency (CIA) had been flying spy missions over the USSR for several years.
My second one deals with South Korean Flight 007 which the Soviet Union shot down killing 269 innocent passengers in 1983. What was not common knowledge at the time of this shoot down was that the United States used the same path for it's spy planes which intercepted Soviet electronic signals. When the Korean Jet drifted into soviet restricted airspace, something that was also not well understood in the west at the time, the Soviet's had reason to believe it was a spy plane. US spy planes had similar speed, radar signatures and flight paths. Also a US spy plane was operating in the vicinity at the same time the South Korean airliner was shot down.
Korean Air Lines Flight 007
The Boeing 747 airliner was en route from Anchorage to Seoul, but deviated from its original planned route and flew through Soviet prohibited airspace about the time of a U.S. aerial reconnaissance mission. The Soviet Air Forces treated the unidentified aircraft as an intruding U.S. spy plane, and proceeded to destroy it with air-to-air missiles, after firing warning shots which were likely not seen by the KAL pilots. The Korean airliner eventually crashed in the sea near Moneron Island west of Sakhalin in the Sea of Japan. All 269 passengers and crew aboard were killed, including Larry McDonald, a Representative from Georgia in the United States House of Representatives.