Error: Invalid data has been used to update the list item. The field you are trying to update may be read only.

I was getting error “Invalid data has been used to update the list item. The field you are trying to update may be read only” when I tried to update list items with incorrect value into People and group field, I was taking value from SharePoint People picker web control and update same values into SharePoint List columns.

Actually we need to pass and assigned value into specific format to people and group data type, below link describes very good and precise information on how to get values from SharePoint people picker control and update into list columns having people data type.

http://dishashah.wordpress.com/2009/06/18/how-to-display-users-to-sharepoint-peoplepicker-and-to-get-data-from-sharepoint-peoplepicker/

Enjoy!!