In the context of a general discussion of the course of action to be followed where an employer subsequently determines that T4 slips issued by it were erroneous, CRA stated:
Where an employer realizes that a mistake has been made, the employer must normally prepare modified T4 slips to correct the situation. Changes can be made via the internet or by producing new paper slips that will be sent to the CRA. In both cases, a copy of the amended T4 slips must be sent to all affected employees so that they can request an adjustment to their income tax return for the particular taxation year. When an employee wishes to obtain an adjustment to an income tax return already filed for a particular taxation year that is within the normal reassessment period, the employee must request that CRA correct the income tax return by filing with the Tax Center, a T1-ADJ - T1 Adjustment Request form, duly completed, or a signed letter that provides all information respecting the request.