Having an incorrect 1257L tax code can result in you paying excessive, or an insufficient amount of tax. A wrong code may be produced because of a change of job, benefits, or possession of various sources of income. In case you think there is an error, then you need to get in touch with HMRC as soon as possible to change your details. They will provide your employer with a restructured code. In case taxes have been overpaid, then a refund can be obtained, and in case of underpaid taxes, then they might have to be paid in the future. By maintaining the correct tax code, you will fulfil your responsibility and you will not have any unpleasant surprises.