In finance, a rogue trader is an employee of a financial services firm, such as an investment bank, who makes unauthorized trades on his employer's account, often covering up the activity with accounting tricks or data mis-entry. Rogue traders' activities only ever come to light when they book enormous losses. Presumably if their unauthorized trades make money, then companies are happy to continue looking the other way.
Some famous rogue traders include Nick Leeson, who bankrupted Britain's venerable Barings Bank in 1995 by losing $1.3 billion betting on Nikkei Index futures, Jérôme Kerviel, who lost $6.9 billion for Société Générale from 2006 to 2008 by placing bad derivatives bets larger than the bank's entire market capitalization, and Bruno Iksil, aka the "London Whale," who lost $6 billion for J.P. Morgan Chase Bank betting on credit default swaps in 2012.