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Where Runaway Losses Come from

Trading books are littered with stories of traders losing one, two, even five years' worth of profits in a single trade gone terribly wrong. Typically, the runaway loss is a result of sloppy money management, with no hard stops and lots of average downs into the longs and average ups into the shorts. Above all, the runaway loss is due simply to a loss of discipline.
Source: http://www.investopedia.com/articles/forex/06/FXMoneyMgmt.asp