From IRS Publication 15 re Employer Tax Withholding:
"Refunding amounts incorrectly withheld from employees. If you withheld more than the correct amount of income, social security, or Medicare taxes from wages paid, repay or reimburse the employee the excess. Any excess income tax or Additional Medicare Tax withholding must be repaid or reimbursed to the employee before the end of the calendar year in which it was withheld. Keep in your records the employee's written receipt showing the date and amount of the repayment or record of reimbursement. If you did not repay or reimburse the employee, you must report and pay each excess amount when you file Form 941 for the quarter (or Form 944 for the year) in which you withheld too much tax."
You cannot recover excess withheld taxes on your return UNLESS it is the result of too much withheld because you had more than one employer during the year. If a single employer errs, you must recover from the employer. Unfortunately, the IRS publication does not indicate what to do if the repayment does not occur in the same year as the error. You need to work this out with the employer if the amount is significant.
