For Collectors: Use TRACES to file Form No. 139 or initiate a correction request.
For Taxpayers: Ensure the TCS is reflected in Form 26AS, then claim it as a credit/refund while filing your Income Tax Return (ITR).
Avoid: Do not expect a refund for "mistakenly paid" challans through the e-Filing portal's general payment services; these must be claimed via ITR or by approaching your Jurisdictional Assessing Officer (JAO).