From ERISApedias text book (which I highly recommend--I found this in no time using their AI feature!).
Corrective Distributions After April 15. If excess deferrals (and income) for a taxable year are not distributed by April 15, the taxation and distribution rules change drastically. First, distribution of the excess deferrals is not permitted after April 15 unless a normal distributable event under Code 401(k)(2)(B) (i.e., age 59-1/2, hardship, termination of employment, or disability) occurs. Second, for tax purposes, undistributed excess deferrals are treated as if they were proper elective deferrals when contributed. This means that they are taxable income to the participant when they are distributed. The effect of these rules is that uncorrected excess deferrals are taxed twice: first in the year of deferral and then when distributed. Excess deferrals that are not distributed by April 15 also are treated as employer contributions for purposes of Code section 415 when they are contributed to the plan. [Treas. Reg. section 1.402(g)-1(e)(8)(iii)]
Example: Suppose that Roberta (from the prior example) did not receive a distribution of her excess deferrals before April 15, 2018. The excess deferrals would remain nonexcludable in 2017, and would be part of Roberta’s taxable income in that year. The $2,500 excess that was already taxed may not be distributed from the plan to Roberta until she experiences a normal distributable event under Code section 401(k)(2)(B).
In 2022, Roberta attains age 59½ and can take a distribution of elective deferrals from the plan. If she removes the $2,500 at that time, it is taxable income in 2022. (The exemption from including the distribution of excess deferrals in income that existed if the distribution is taken by the following April 15 evaporated when the distribution was not timely made.) As a result, Roberta is effectively taxed twice on this amount – once in 2017 and once in 2022.