fat finger trade
n. An erroneous or inadvertent trade, particularly one that has significant market consequences, caused by a typo. Also: fat finger, fat-finger trade, fat fingers trade.

Example Citations:
A trader at RBS has admitted to making a fat finger trade in the EUR/CHF pair Monday, a spokesperson for the bank told Reuters...Initial reports had claimed that it was RBS's algorithms that caused the spike. However, those reports were later proven wrong. It turns out that it was a simple fat finger trade that caused the spike and algos reacted to send the pair even higher.
—Matthew Kanterman, "Another Day, Another Trading Glitch: RBS Admits Responsibility in EUR/CHF Spike," Benzinga, August 9, 2012

Gold futures ended nearly unchanged Monday, after a large early-morning sell order roiled traders and slashed prices by almost $15...Gold traders buzzed with speculation that the transaction was an input error — a so-called "fat finger" trade...One indicator that the transaction was a mistake was its size. At 750,000 troy ounces, such large trades are rarely conducted amid very thin trading volumes.
—Tatyana Shumsky, "Gold Shakes Off $1.24 Billion 'Fat Finger'," The Wall Street Journal MarketBeat, April 30, 2012

Earliest Citation:
Vodafone's initial fall led telecoms lower. Trading volume in the mobile phone giant was inflated by an incorrectly reported trade, which had been withdrawn and subsequently been re-entered, the London Stock Exchange said..."This was a real fat fingers trade," said one trader.
—Camilla Reed, "UK's FTSE creeps up at midsession, wants U.S. lead," Reuters, April 19, 2001

Notes:
Many thanks to Twitter user @rur42 for letting me know about this phrase.

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