What should you do if you ended up paying more than allowed amount in 401K?

If you paid more than the allowed amount on your 401(k), contact your employer 401(k) rep about the amount that you have overpaid. You should receive a check from the 401(k) company or your plan administrator. This amount is to be reported on the Tax Form 1040, line 7. Since this money went into your 401(k) untaxed, it now has to be taxed. Report this money in the tax year you receive it.