What can an employee do if your employer forgot to include per diem non tax amount on your salary? Can the employee still file that amount on their return with employer backup?

I guess it depends; under an accountable plan, amounts paid under accountable plans are not income to the employee and are not shown on Form W-2. The employee must adequately account to the employer for these expenses within a reasonable time.The employee must return an excess reimbursement or allowance within a reasonable period of time. However, under the non-accountable plan, Amounts paid are income to the employee and must be included in wages with appropriate tax withholdings.An employer can have an accountable plan for some items, and a nonaccountable plan for others.