[hga]

There have of course been some since the 2008 credit crisis and the various bans on short selling and according to The Wall Street Journal the effects weren't good (but take that with a big grain of salt). The example of commodity onion trading is supposed to be the classic one showing it's a bad idea.

[elecengin]

There is no ban on "shorting" onions specifically - it is just not traded on the futures market at all. Gerald Ford pushed through legislation on behalf of onion farmers in the late 1950s to disallow it on the futures market. It has been an example of extreme market volatility ever since.