Shares in HSBC jumped nearly 10% in a matter of minutes in what traders said looked like a costly mistake.
The bank jumped from around 630p to 688p, prompting an automatic suspension of the shares for five minutes.
When they restarted they fell back to their previous level. One trader said: "This looks like a fat finger, it's a complete error. It looks like someone has taken a hit of around £400,000."
The London Stock Exchange confirmed the temporary suspension, and said it was investigating the situation. Depending on the outcome the trades could be allowed to stand, or could be cancelled.
The move in HSBC briefly pushed the FTSE 100, which had been weaker on emerging market worries, into positive territory.
Earlier an 11% drop in drinks giant Diageo was also put down to a "fat finger" mistake, when a trader accidentally inputs the wrong price for a buy or sell order into their terminal.
With high speed trading commonplace, such mistakes are not difficult to make. In 2001 a Lehman Brothers dealer in London keyed in £300m for a trade in GlaxoSmithKline and AstraZeneca which should have been £3m.
