Comets have long been the archetypal herald of the apocalypse, and Halley's Comet in particular has caused predictable hysteria every 75 years, but one visitation stands out from the others.
In 1910, as the comet drew close to Earth, astronomers at the Yerkes Observatory of Chicago announced that they had studied the comet's tail and discerned that cyanogen (oxalyl cyanide) gas was present. This discovery didn't receive much attention until an amateur chemist began propagating the idea that the cyanogen would react with the oxygen in the atmosphere as Earth passed through the comet's tail. The two gases would combine into nitrous oxide (colloquially known as laughing gas) and the world's population would die in fits of horrible, choking laughter.
Fear burned through Chicago and spread out into the rest of the nation. The Yerkes Observatory tried to dispel the doomsday rumors, but religious doomsayers brought up "Wormwood," a fallen star mentioned in Revelations that was prophesied to poison the Earth. This wasn't terribly helpful. In a desperate attempt to keep out the gas, people tried to seal shut their homes using tape and newspaper. Other panicked citizens bought crude gas-masks and phony "comet pills" from unsavory figures out to exploit the situation. One Georgian man decided to lower himself to the bottom of a forty-foot well with a gallon of whiskey.
There were also those who decided to embrace the end of the world in style. By the time May rolled around and the comet became visible, numerous roof-top comet parties were held. One wonders if they were faintly disappointed when, inevitably, the world continued. The headline of that morning's Chicago Tribune was "We're Still Here!", needlessly alerting people to the fact that they were still alive. It seems the astronomers at Yerkes Observatory had the last laugh.
But don't take my word for it!
You can read about it in a book