According to a study led by Michigan State University professor Mark Skidmore, some $21 trillion in Pentagon accounting transactions, made over a 17-year period, lack supporting data. This doesn't mean the money is gone — the Pentagon only received roughly $9.2 trillion in budget money during that time — but it does mean the system is so choked with bad numbers, we have little idea of what mischief might be taking place at the Department of Defense.

Here's a brief list of some of the problems likely lurking in the Pentagon's books.

Double-Billing

An alarmingly simple scheme. As Andy the Air Force accountant puts it: "A contractor accidentally invoices the military twice. It gets paid twice." For example, one company got paid in full for delivering just 28 percent of a lug-nut contract; or the Swiss contractor that supposedly over-billed by $757 million to supply the troops in Afghanistan.