Tds deducted but no money recived

Mr. A was preparing his Income Tax return. He knew that he had received 3 incomes from different sources on which TDS has been deducted. Just in order to maintain accuracy before furninshing return, he checked his 26AS and to his surprise he noticed in 26AS that an amount of 30,000 has been deducted too as TDS on fourth source of income. He neither recived such income (fourth source) nor is aware about it.

What should he do? and how could this happen? what are the legal consequences, if any?