The Pentagon's top watchdog of efforts to rebuild Afghanistan has uncovered a pile of instances of U.S. taxpayer dollars going to waste over the past 15 years. But on Thursday, he said that some failures in reconstruction could have actually killed American troops.

In one case, the U.S. spent about $32 million to install steel bars in the pipes that allow water to flow under roads to prevent insurgents from placing bombs in them.

However, the inspector general found that two Afghan contractors in one province either improperly installed, or in some cases never placed at all, about 250 of these systems.

"U.S. troops may have died or suffered wounds as a result," John Sopko, the special inspector general for Afghanistan reconstruction, said in remarks prepared for a speech Thursday afternoon at Harvard University.

Since 2002, Congress has appropriated more than $113 billion to rebuild Afghanistan, more than the Marshall Plan to rebuild Western Europe after World War II when adjusted for inflation.

In another instance of procurement gone wrong, the Pentagon spent $486 million to buy 20 second-hand G222 cargo planes from Italy for the Afghan Air Force before it realized that the planes had not been properly refurbished and would require parts for maintenance that were no longer available.

Sixteen of the planes were later sold as scrap metal for six cents a pound for a total of $32,000.

Sopko said it's important to examine successes and failures to be better prepared the next time the U.S. is involved in a conflict where rebuilding is necessary.

"The world is a dangerous place, and the future is likely to include new 'contingency operations' involving American military and civilian personnel in violent settings that will entail rebuilding failed or fragile nation-states," he said. "Close examination of operations like the one that began in Afghanistan nearly 15 years ago can help us make more informed decisions and, hopefully, produce better outcomes in the future."