What happens if I contribute too much to my 401k?

If you over contributed (made excess deferral contributions) to your 401(k), you should notify your employer or the plan administrator immediately. Ideally, this notification should be provided by March 1 of the year following the year the excess deferral occurred. The excess deferral amount should be returned to you by April 15. Excess deferral amounts returned to you should include earnings accrued on the excess amount while it was in your 401(k) plan account. You are required to add the earnings to your taxable income for the year the excess amount is distributed from your 401(k) plan account. In addition, if the excess amount was deferred on a pre-tax basis, your employer must amend your W-2 to show the returned amount as wages.

SenseFinancial.com