A serious incident report was emailed to the wrong care home in one of 18 data losses recorded at Sheffield Council.
Information security incidents also included confidential paperwork about mental health patients being found in the street and then handed in to The Star.
Data, revealed under The Star’s Your Right To Know campaign, shows documents about staff attendance were found in a public place and a support worker’s visit schedule and key safe information was left in someone’s home.
Paperwork with the first names of children and possible equipment needs for schools to help with physical access were reported missing, as was a file containing financial information and staff payment details.
The council - which started recording such data in February 2012 - referred two incidents to the Information Commissioner.
They included the case involving mental health patient paperwork, when the commissioner made recommendations.
And a second incident where 1,729 staff who had made work purchases of over £500 had their names published online was also referred.
That spreadsheet was amended and no further action required.
A council spokeswoman said ‘any loss is a loss too far’ but that data lost was small compared to the size of the authority.
She added: “This council has more than 6,000 employees handling hundreds of thousands of records on a daily basis. We have never been fined by the Information Commission, when various other authorities have. We have even self-referred ourselves to the ICO.
“Over three years we had 18 incidents where regrettably individual staff have lost or leaked information.
“We have in the majority of incidents immediately redacted or recovered this and have informed individuals where information has been lost.
“I would like to assure people we go to great lengths as a council to protect data.”