Google has an annual contest for kids called Doodle-4-Google. But for submissions to be accepted this year, Google required parents to submit their child's city of birth, date of birth and the last four digits of their Social Security number.
You see what Google knows and many parents don't know is that a person's city of birth and year of birth can be used to make a statistical guess about the first five digits of his/her social security number. Then, if you can somehow obtain those last four SSN digits explicitly - voila, you've unlocked countless troves of personal information from someone who didn't even understand that such a disclosure was happening.
NY Mag's Daily Intel blog has an official comment from Google explaining their motives:
This year we started accepting doodles from kids even if their school hadn't registered for the contest. To help us keep entries distinct and remove duplicate entries from any particular student, we asked parents for limited information, including the last 4 digits of a student's social security number. We later updated our forms when we recognised that we could sufficiently separate legitimate contest entries while requesting less information. To be clear, these last 4 digits were not entered into our records and will be safely discarded.