Earned Income Tax credit (EITC) has a lot of fraud connected with it, maybe $16 billion per year.

In this article from taxprof blog: 

For the third consecutive year, the IRS did not publish annual reduction targets or report an improper payment rate of less than 10 percent for the EITC . . . although risk assessments were performed for each of the programs that the Department of the Treasury required the IRS to assess, the risk assessment process still may not provide a valid assessment of improper payments in tax administration. As such, the EITC remains the only revenue program fund to be considered at high risk for improper payments.