Probably everyone has accidentally left some papers in a shared printer at one time or another. But when papers containing sensitive personal data get scooped and included in a mailing to an uninvolved their party, you may wind up violating the Data Protection Act.
According to the Information Commissioner’s Office, that’s pretty much what happened to the City of York Council, who reported their error to the ICO on February 10, and who subsequently signed an undertaking to prevent a recurrence of the problem.