From Chris Blattman, citing an ODI study which I’m still tracking down:
The largest documented case of fraud in a humanitarian program providing money is from the United States. In the wake of Hurricanes Rita and Katrina, the Federal Emergency Management Agency (FEMA) quickly provided aid through the Individuals and Households Program, which provided money for housing and immediate needs.
As of February 2006 more than 2.6 million payments were made totalling over $6 billion. The US Government Accounting Office estimated $1 billion of these payments were fraudulent from bogus claims and double registration (GAO, 2006).