MF Global Funds Traced? How were they lost?

So MF Global has finally traced the $1.6 Billion dollars they lost. I won't go into detail about the article. I just have one question and if you are a company, how do you lose track of $1,600,000,000? That's right, I did not want to write billion, I want to elaborate on the 0's. Furthermore, how does a company such as JPMorgan not pick up on the fact that they have some of that missing money? What is going on with these investment firms?