If you get too much change back from a big chain store, you’re likely to keep it. Defense contractors are no different.

One contractor was able to keep a $7.5 million overpayment for eight years because of Department of Defense billing problems, according to a recently released General Accounting Office (GAO) report entitled Fixing DOD’s Payment Problems Is Imperative. GAO estimated that the overpayment cost the government nearly $5 million in lost interest. More importantly, TC$ worries that some overpayments may never be discovered.

The report states that until DOD effectively reforms its payment process “it continues to risk hundreds of millions of dollars in potential overpayments and other financial management and accounting control problems.” In addition to overpayment problems, DOD’s billing is beset by inefficiencies including time wasted maintaining separate billing codes for closely related items, matching documents manually and computer system incompatibility.

DOD also maintains misleading and needlessly specific records. For example, maintaining separate accounting reference numbers for dinosaur pencil tops, pumpkin banks and Halloween pinball games for a $1,209 supply contract for a Navy child care center consumed excessive time and resources according to the GAO.

In comparison to other non-federal billing contractors, DOD’s billing system is highly inefficient. Processing a contract invoice costs DOD 14 times more than the private sector in direct labor costs. An average private sector contract invoice can be processed for $3 in direct labor costs, but for DOD, it costs $44. A private sector worker can process 16,400 invoices a year, while the average individual at DOD processes about 1,000 invoices per year.

Not only is DOD’s payment process inefficient, but the extra time doesn’t seem to help. The GAO found that in a sample of DOD checks returned by contractors, 78 percent of $392 million worth of checks were overpayments by the government.