An investigation has been launched after the personal and financial details of more than 50 people were mistakenly sent to a training company.
The package was addressed to Revenue and Customs
The sensitive files should have been delivered to the HM Revenue & Customs office in Cumbernauld.
However, they arrived at the headquarters of Insights Learning and Development Ltd in Dundee.
The papers contained ages, addresses, National Insurance numbers and banking details.
A spokeswoman for HM Revenue & Customs confirmed enquiries were under way to establish how the mix-up occurred.
She said: "We are taking this matter very seriously and are carrying out a full investigation."
The organisation said it was too early to say where the package had come from.
Michael Simpson, a production assistant with Insights, said the package was addressed to the HM Revenue & Customs base at Cumbernauld.
He said: "When we received the package it was quite badly damaged, which in itself was not right.
"We had a look to see what it was and discovered all these details.
He added: "When we spoke to the Inland Revenue and the courier company they very casually asked of we could re-address the package and they would pick it up for re-delivery.
"We felt it was in the public interest to make people aware of this because it is the public's personal privacy that has been invaded."
A spokesman for courier company TNT said the package had been picked up for re-delivery to Cumbernauld.
The mix-up comes just days after the Scottish Government confirmed a package containing the personal pension details of 200 people had gone missing in the post.
It was tracked down at Stobhill Hospital in Glasgow at the weekend, four weeks after the documents were last seen.
Concerns over government handling of sensitive information were first sparked last week with the loss of child benefits discs, which contained the personal details of all families in the UK with a child under 16.
The Scottish Government is expected to review its handling of personal data.