This secret Russian weapon turned out to be harmless bee poop
In 1981, the United States accused the Soviet Union of providing chemical weapons to Communist countries in Southeast Asia to test a new kind of toxin for use in the ongoing arms race. Hmong tribesmen who fought with the US and South Vietnamese before the end of the Vietnam War described a kind of "yellow rain" pouring down on them from the skies. The truth was a relief to both the villagers and the bees – and was actually kind of worse, in some ways.
Today is the day you readers learn that bees poop.
Refugees fleeing the reprisals from the Communist governments of Laos and North Vietnam reported to their American allies that low-flying helicopters had dumped an oily yellow liquid on them en masse. Those hit by the liquid claimed to have suffered from seizures, headaches, and blindness, as well as some internal bleeding. The United States didn't know exactly what the substance was, but they believed it was a chemical agent the Soviets were testing on these unwitting refugees.
President Reagan's Secretary of State Alexander Haig accused the Soviet Union of doing as much in 1981:
For some time now, the international community has been alarmed by continuing reports that the Soviet Union and its allies have been using lethal chemical weapons in Laos, Kampuchea, and Afghanistan. ... We have now found physical evidence from Southeast Asia which has been analyzed and found to contain abnormally high levels of three potent mycotoxins—poisonous substances not indigenous to the region and which are highly toxic to man and animals.
The Soviets, of course, denied every word of this. They would have denied it even if it was true. And you can't blame Haig, this totally sounds like something the Soviet Union would do. But to get the international community behind the accusation, the United States needed to prove it – that would prove to be a huge problem.
The refugees were inconsistent with their stories, and the so-called samples of the agent proved inconclusive in laboratory testing. Furthermore, the sufferers of the effects of the "toxin" were actually suffering from fungal infections in their skin. No one knew exactly what was happening until a Harvard researcher decided to check it out once and for all. Biologist Matthew Meselson traveled to the affected areas to do his own legwork, separate from the official investigation. Traveling to Laos, he recreated the events surrounding the alleged attack and found that it wasn't a strange chemical substance. It wasn't even strange. In fact, it was fairly common.
Because everybody poops.
Meselson's team found the yellow raindrops clinging to leaves in northern Laos. Once they concluded that this is what the refugees were describing, they conducted a test on the matter and found many kinds of pollen grains in it, typical of those digested by bees. It was, in fact, bee poop. Huge clouds of bee poop fell on those people.
When his study went up for peer review, it was found that villages in China had the same experience, only instead of chemical weapons, the villagers thought the yellow rain foretold a dangerous earthquake.