In 1955, more than 30 million of pounds of onions were sent to Chicago. Two traders at the Chicago Mercantile Exchange, Sam Siegel and Vincent Kosuga, were buying up every onion they could find. They bought not only freshly harvested onions but also onion futures (the right to buy onions at a fixed price in the future).
Eventually they had cornered 98% of the onion market.
They went to onion buyers and were able to set their own prices because they had assumed a short term monopoly on onions. They sold onions back to growers by threatening to flood the market with onions otherwise.
When their onion supply dwindled, They did all sorts of tricks to make it seem like there was a surplus of onions. At the same time they were shorting onion futures. They got drove onions down from $2.75 to 10 cents per 50 pounds.
The government eventually passed the Onion Futures Act in 1958 that prohibited onion trading and excluded them from the definition of a commodity. In 2010, the law was extended to include box office futures.
All systems eventually get exploited and all channels eventually get saturated. Hopefully those systems patch themselves in a reasonable way (do onions really need to be singled out here?). There won't be another opportunity to corner the onion market like Siegel and Kosuga did in 1955.
Fun fact: Chicago is named after Chicagoua, the name that Miami-Illinois Native Americans used for "stinky onion" which was actually a garlic plant that grew along the Chicago River.