HM Revenue & Customs has apologised after accidentally distributing private details about thousands of tax credit claimants.
Individuals were apparently sent other people's personal information in the post along with their annual award notice.
One woman from Hyde in Greater Manchester told the BBC she had received a letter that included her neighbour's earnings.
She also got the bank sort code and the last four digits of the bank account number of another claimant.
An HMRC spokesman blamed a printing error, and played down the danger of identity theft.
"HMRC takes data security extremely seriously," he said. "Unfortunately an error has occurred in one of the tax credits print runs causing some customer information to be wrongly formatted.
"Investigations are under way to identify the cause of the problem and we will be contacting affected customers in writing this week, apologising and providing a corrected award notice.
"An initial analysis shows that ID theft could not result from this printing error."