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The Department of Defense's failure to act on recommendations from its own watchdog may have cost $33.6 billion, a new report says.

The Pentagon's Inspector General's Office released a 458-page report last week that detailed how the Department of Defense responded to 288 Inspector General audits that went back as far as 2006. The report concluded that the Pentagon addressed very few problem areas and may have cost itself and American taxpayers $33.6 billion in wasteful spending.

The costliest example provided in the report listed the Marine Corps purchase of the CH-53K helicopter. The military branch planned to buy 44 more helicopters than it needed in 2013, according to the IG, costing taxpayers an additional $22.2 billion.

The Pentagon is seen in this aerial view on March 27, 2008. (File) Reports of sexual assaults in the military increased slightly last year, U.S. defense officials said Monday, May 1, 2017, and more than half the victims reported negative reactions or retaliation for their complaints. Charles Dharapak / AP

Despite the IG's warning against the purchase, the Marine Corps went ahead with the acquisition.

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The Department of Defense did not respond to requests for comment.

Some of that money can still be recovered if the department follows 58 of the IG’s suggestions that contain “associated potential monetary benefits,” though the report, released Thursday, notes that a number of those opportunities may have passed.

“We believe that [the Department of Defense] senior managers should focus attention on the 1,298 open recommendations and ensure that prompt resolution and action is taken,” the report said.

The Pentagon “agreed to take corrective action on 1,251” of the recommendations, according to Acting Inspector General Glenn Fine.

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As of March 31, when the office concluded its overview, 832 recommendations had been pending for more than a year, 109 had languished for more than three years and two had gathered dust for more than a decade.

The second-highest dollar amount in waste came from the Air Force's purchase of $8.8 billion worth of MQ-9 Reaper drones. The IG said it was a waste as the Air Force had spent the same amount on the drones in previous years.

Despite the admitted losses, the report also made a few suggestions that could save the Department of Defense money in the future — if the recommendations are followed. Some of that information, particularly reports related to the F-35 program and Cyber Command, was classified and redacted from the public version of the report.

One example of a problem that the IG’s office said is fixable but remains unaddressed since its audit is how Tricare, the military health insurance, is managed overseas. An April 2014 audit concluded that the insurance provider pays its overseas contractors whatever it is billed instead of negotiating rates. This mean that $21.1 million in 2009 payments ballooned to $63.8 million in 2013.

Of the service branches: the Army maintains 274 open recommendations; the Air Force hasn’t addressed 166 recommendations; and the Navy carries the fewest at 148.