Fat Finger
A fat finger is a trading error caused by simply typing or clicking the wrong thing when entering an order. It's not a strategy mistake or a misjudgment of the market — it's a mechanical slip, like typing the wrong ticker, adding an extra zero to the share quantity, entering the wrong price, or hitting "buy" when you meant "sell."
It happens because order entry screens ask a trader to fill in several fields quickly — symbol, side, quantity, order type, price — often under time pressure. A single wrong keystroke or misclick can turn a routine order into something very different from what was intended. Because trading platforms generally execute orders exactly as entered, with no way to know what you "meant," the mistake usually only becomes visible after the fill.
The nuance that trips people up is scale. A fat finger on a small personal account might just be an embarrassing, cheap lesson — buying the wrong stock or the wrong quantity and having to unwind it. But the same kind of error at an institutional trading desk, where orders can involve large size, has occasionally moved prices sharply within seconds and triggered brief, unusual price swings across a market before anyone could react. Exchanges and brokers have built in some safeguards, such as order size limits or price-collar checks that reject orders too far from the current market price, but these do not catch every case.
It's also worth distinguishing a fat finger from a bad decision. If you buy a stock and it drops because your analysis was wrong, that's not a fat finger — that's a normal trading loss. A fat finger specifically means the order that executed does not match the order you intended to place.
This term depends on a rule or threshold that changes over time, so no specific figure is quoted here. The definition mentions exchange/broker safeguards like price-collar checks and order size limits in general terms without citing specific thresholds, so no number needs sourcing here. However, if a specific historical fat-finger incident or a specific exchange rule/threshold (e.g., a named circuit breaker percentage or a specific price-collar band) is later added to this entry, that figure should be confirmed against current exchange (e.g., NYSE, Nasdaq) or regulator (SEC/FINRA) rule documentation, since these mechanics and thresholds are periodically revised.
A day trader moves fast and re-enters orders constantly, so the odds of a costly slip — wrong size, wrong symbol, wrong side — are higher than for a buy-and-hold investor, and there's often no time to review before the fill hits.
Dan means to buy 100 shares of $FSLR at 79.90 but instead types $FSLY and 1,000 shares at 79.50. The order fills instantly on the wrong stock at ten times the intended size. He now has to notice the error, sell out of the accidental position, and separately place the trade he actually wanted — all while eating the price difference on both.
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