Soviet citizens lived in a world where all media was controlled by the government and therefore suspect. Also, since the government had indeed killed or imprisoned many people for political reasons it wasn't unreasonable to suspect the government in an important person's death. Trotsky, one of the founding members of the Soviet Union, was assassinated by Stalin, not to mention many of Stalin's generals and other officials. In this context, suspicion was entirely rational.

I can think of two important people whose death was widely suspected to be caused by the government when most signs point to them being accidental. The first is Yuri Gagarin, first man in space, who was killed in a fighter training accident some years later. There is a popular theory that he was purposefully killed by the government.

Another is Vladimir Visotsky, an actor and musician who by all accounts had serious drug and alcohol problems. However he had snubbed the authorities in numerous ways, writing subversive songs, traveling without official permission and so on, and so it was entirely expected that the government might try to do away with him.

While this is not strictly a soviet period conspiracy I recommend reading about the Moscow apartment bombings to get a feel for the level of suspicion and uncertainty surrounding any story told by the Russian government. In short there is a varying level of belief regarding whether all Chechen terrorist acts in the '90s-'00s were a government conspiracy. This includes the 1999 bombings that provided the excuse for the second Chechnya war, the Moscow Theater hostage crisis where the government used poison gas to subdue the attackers and killed many of the hostages, and the Beslan school attack where again actions by the authorities resulted in the deaths of many hostages.

Not from the USSR but from Hungary, a former communist state. As media was entirely controlled by the state there were a lot of rumors circulating some still remain entrenched to this day. A popular one was that tobacco Industry was developed because in the event of a war the cigarette loading machines could be used to manufacture ammo. Another one was that canned foods are all round and are made only in specific sizes all through the eastern bloc because these machines can be used to manufacture artillery shells. Another popular one was that maps are intentionally inaccurate to make it impossible to navigate abomber according to official maps. It did not help at all that military objects were usually represented as wineries, forests or agricultural lands on official maps. And there were no other kinds. We had a recurring theme, as Soviet troops were stationed in Hungary, you could meet them from time to time however the official numbers were way distorted and a bit contradictory. So everyone had their own guess how many Russians are in the country.

As a widespread spy network was operated by the government it was a pretty widespread game to forge theories about who are writing reports for the secret service. This one continues to this day as the old records are still kept secret so from time to time a new conspiracy theory springs up that this or that famous politician or artist was writing reports to the secret service about his friends. Sometimes this is backed up by data but many times just stays as a wild rumor.

We had a revolution in 1956 which was the source of conspiracy theories for decades, some new ones popping up even nowadays. You could hear stories about secret underground dungeons where people were beaten to death and then ground up with a meat-grinder and dropped into the river. There were even fishers who testified that in the late fifties at a certain point of the river Danube the catch was always plenty. There were rumors about whole chambers filled with cadavers and then hid with a fake wall where tenants would smell something rotting for decades. There are wild rumors to this day about the people executed after the revolution depending on your worldwiev. Some people argue that most of the revolutionaries were in fact common thugs some argue that they were CIA agents.

I can look for some more if anybody is interested.

Follow up / Extension:

If not, why? Media? Culture? I'd be happy to hear the thoughts from someone who has greater insight into the minds of Soviet citizens or the origins of these 'theories' in the US.

How popular was the Lost Cosmonaut conspiracy in the USSR?

In the 30s, there was a conspiracy theory that assassination of Kirov, leader of Leningrad's communist party, was organized by Stalin and NKVD. Now, a lot of historians agree with it, but without definite proof. There even was a famous rhyme (chastushka), «Эх, огурчики, да помидорчики, Сталин Кирова убил в коридорчике», which can be roughly translated to "cucumbers-tomatoes (nonsense like this is typical in this genre as long as it rhymes), Stalin killed Kirov in a hallway".

http://www.historytoday.com/blog/books-blog/david-priestland/kirov-murder-and-soviet-history

Also, events like Novocherkassk massacre and Chernobyl incident were first conspiracy theories, which were later completely confirmed. Unfortunately, I couldn't find sources that had words "conspiracy theory" in them, but all sources about these events stated that information was suppressed, so it seems obvious that they would be discussed in that manner at the time.

Not exactly an answer to your question, but since you brought it up and are interested in the Soviets, you might be interested to know that according to the "Mitrokhin Archive," the popular JFK conspiracy theory was fabricated and spread by the KGB.

The "Mitrokhin Archive" is the collected hand written notes allegedly copied by KGB defector Vasili Mitrokhin who was an archivist for the state security bureau.

It's impossible to verify whether his notes were genuine or not, but they contain some incredibly fascinating things. Along with the JFK conspiracy, the KGB also apparently spread the conspiracy theories that AIDS and the MLK Jr. assassination were acts of the US government, as well as helped to spread the rumor that J. Edgar Hoover was gay.

You can check it out here: http://digitalarchive.wilsoncenter.org/collection/52/mitrokhin-archive

It isn't widely known, but the Mitrokhin Archives exposed the fact that the conspiracy theory that JFK had been killed by his own government was in fact fabricated, launched, and pushed by the KGB.

"In September 1961 the CPSU Central Committee allocated another 55,000 dollars for the next two years to allow Marzani to expand his publications. He was given a further 10,000 dollars a year to cover advertising costs.19 When the young KGB officer Oleg Kalugin, stationed in New York in the early 1960s under cover as a Radio Moscow reporter, paid his first visit to one of Marzani’s receptions, he found his apartment “filled with a motley assortment of Communists, liberals, and KGB spooks-all of them watched, undoubtedly, by FBI informers in attendance.”

Among the books published by Marzani in 1964 was the first volume on the Kennedy assassination to appear in the UnitedS tates, Oswald: Assassin Fall-Guy? by the German writer Joachim Joesten. At the beginning of the book Joesten expresses his “heartfelt thanks . . . to Carl Marzani, a shrewd and hard-hitting publisher in the finest American tradition, who put his whole heart and soul in this book;” Marzani succeeded in publishing it within five weeks of receiving the manuscript. 21 Joesten supported Moscow’s line in pinning the blame for the assassination on a conspiracy by right-wing racists, chief among them “oil magnate H. L. Hunt:”

The KGB was also directly responsible for the conspiracy theory that the U.S. had faked the moon landings, and that the CIA introduced crack-cocaine to the African-American community.

These were what the KGB called "active measures", and they managed quite a few during their time. Those stick out as measures that are still "popular" today. When those measures took hold, Soviet propaganda used those beliefs abroad, and in the U.S. to undermine U.S. government position or efforts abroad. For example, they'd talk up the fact that the crack-cocaine conspiracy "was widely believed" in the African-American in the international communist schools, which hosted a large number of African students.

Source:

The Sword and Shield: Mitrokhin Archive and the secret history of the KGB, Andrew and Mitrokhin, chapter fourteen.

People are referencing the Mitrokhin "archive" here as if it is a cast-iron source. In reality, it seems quite controversial.

If someone could put forward an argument for why that stuff should be taken at face value, I would appreciate it.

I'm late to the party, but there is a great read that speaks to one conspiracy theory during the end of the Soviet era. Putin accumulated power during a series of fatal terrorist bombings on residential apartment buildings. The theory suggests that Putin is directly responsible for the bombings and used the threat of more bombings as leverage to gather power, which eventually lead to his current position. It's pretty well researched and really creepy.

Why were so many of those comments removed?

I don't know about populations from other SSRs, but in both Georgia and Armenia, there is a (continuing to today) widespread belief that Stalin developed a "secret weapon" that enabled the Soviet military to cause earthquakes to happen wherever they chose around the earth.

This was explained to me by various people as the cause of Japanese tsunami and subsequent Fukushima meltdown (punishment for a territorial dispute) and the massive earthquake that struck Armenia's northern region (specifically Gyumri/Leninakan) in 1988.

Answering in an old thread:

in some countries: Poland, Czech republic - there was a fear of a black volga (type of a car), whose driver or passengers would kidnap children. Volgas were usually owned only by high ranked officials or soviet advisors, as well as secret police, so this whole rumor can be based on truth - maybe the secret police kidnapped someone in plain day?

A follow-up question... A long time ago my family's synagogue, like many American synagogues, sponsored a Jewish family to come to the U.S. from the USSR. They faced terrible persecution there, so I think basically they were treated as asylum seekers for immigration purposes.

My question is this: did Soviet Jews face persecution on the basis of conspiracy theories about Jews? If so, what kinds of theories were propagated?