If your 401(k) contributions exceed the limits above, you may end up being taxed twice on your excess contributions: once as part of your taxable income for the year that you contribute and a second time when you withdraw from your plan. Earnings still grow tax-deferred until you withdraw them.
If you realize you contributed too much to your 401(k), notify your HR department or payroll department and plan administrator right away. During a normal year, you have until your tax filing deadline—usually April 15—to fix the problem and get the money paid back to you.
Excess deferrals to a 401(k) plan will have to be withdrawn and returned to you. Your human resources or payroll department will have to adjust your W-2 to include the excess deferrals as part of your taxable income. If the excess deferrals had any earnings, you will receive another tax form that you must file the following tax year.