Rogue trader

A rogue trader is an authorised employee making unauthorised trades on behalf of their employer. It is most often applicable to financial trading, and as such is a term used to describe persons - professional traders - making unapproved financial transactions.
This activity is in the grey area between civil and criminal illegality for the reason that the perpetrator is a legitimate employee of a company or institution, yet enters into transactions on behalf of their employer without permission.
One famous rogue trader is Nick Leeson, whose losses were sufficient to bankrupt Barings Bank in 1995 following his ill-advised and unauthorized investments in index futures. Through a combination of poor judgement on his part, lack of oversight by management, a naive regulatory environment and unfortunate outside events like the Kobe earthquake, Leeson incurred a $1.3 billion loss that bankrupted the centuries-old financial institution.