Tax credits are designed to help working families on low incomes
Child tax credits and working tax credits were meant to be Labour's flagship welfare policy - designed to help hard working families on low incomes make ends meet.
But since they were introduced two years ago, an astonishing 1.8 million people have been paid the wrong amount.
Many are now suffering financial hardship as overpayments are clawed back - with some facing the threat of losing their homes.
HELPLINE: Working Tax Credit/ Child Tax Credit
8am - 8pm seven days a week
Great Britain: 0845 300 3900
Northern Ireland: 0845 603 2000
See internet links for low income charities, below right
And, as Paul Kenyon reports, the rest of us may have to pick up a bill of £ 2.2 billion if incorrect payments are eventually written off.
According to the government, claimants such as those in our programme should have realised that they were getting too much and put the extra money to one side for repayment.
But we reveal how not even Oxford University's finest brains can fully understand a typical tax statement.
Real Story: BBC ONE on Monday 14 November at 1930 GMT.
Unfortunately, this edition of Real Story is not available to watch on the website because of copyright issues. We're unable to provide viewers with tapes for the same reason.