Unless timely distributed, excess deferrals are (1) included in a participant’s taxable income for the year contributed, and (2) taxed a second time when the deferrals are ultimately distributed from the plan. See IRC Sections 402(g)(1) and 402(g)(2) and Reg. Section 1.402(g)-1(e)(2). A participant who fails to receive a distribution of the excess deferrals does not receive basis in his pre-tax deferral account equal to the amount of excess deferrals. See IRC Section 402(g)(6).
https://www.irs.gov/retirement-plans/consequences-to-a-participant-who-makes-excess-deferrals-to-a-4...
Just amend your tax return and go over each section again, changing your answers as needed. When you review your W-2s, Turbotax should notify you that you appear to have excess 401(k) deferrals (they should be listed on your W-2s in box 12 with code D). Change your answer from "I will remove the excess" to "I won't remove the excess." The tax should be recalculated and will lead to extra tax being due when you submit the amended return.
