Nick Leeson and Barings
Leeson was the trader who managed to bring about the collapse of Barings Bank in 1995. The main reason he was able to do this was because there was a central weakness in the banks internal controls being a lack of segregation of duties. Leeson was responsible for entire Singapore office (trading, back office and management) and was able to hide huge trading losses in the accounts whilst reporting healthy profits to his superiors in London.
Other weaknesses in the controls lied with his superiors in London who didn't properly interrogate and review the accounts and simply just accepted them for what they showed, and even gave Leeson bonuses for falsified profits in accounts. Furthermore the auditors failed to bring to light these basic weaknesses in internal controls.