I disagree, these sort of losses are usually caused by failures in risk management systems or procedures. Nick Leeson was permitted to settle his own trades while being head of a trading desk (which is never a good idea), and Jerome Kerviel had previously worked in the middle office at Societe Generale and used his knowledge of their systems and procedures to hide his trades. Kweku Adoboli had previously been a trade support analyst (a middle office position) and UBS is currently undergoing a major restructuring of its risk management systems, so it wouldn't surprise me at all if he were able to circumvent their systems to escape detection. The universe seems to be telling us that middle office people shouldn't be moved to front office positions on a delta one desk.