Accidentally contributed to pretax 401K

[noviceattax]

Dear Community,

I accidentally contributed to pre-tax 401K from one pay check. In my memory, I didn't do it. It is some issue with the portal. All my 401K contributions have been after tax Roth so far and I want to keep it the same way even now.

As soon as I noticed that this mistake has happened I notified my employer and the broker and they said that it will be fixed but I saw it is not fixed yet and now they are saying that they can't fix it. 

Do I have any option to resolve this?

I was thinking about the following option to my rescue:
Context: This year I have changed my employer. I was contributing to 401 K even with my previous employer. So my new (current) employer relies on me to tell when my contributions have reached the limit. I am thinking to use this as leverage with the following plan. 
Plan: I will let my this year's contributions go over the limit (new employer's pretax + new employer's Roth + previous employer's Roth). I will do this by contributing more in after tax Roth with the new employer. I will do it such that   new employer's Roth + previous employer's Roth meets the limit. But when new employer's pretax is considered I will be over limit. So when it will come to fixing the over limit, I will withdraw the money from pre-tax section?
My questions:

1. What do you think about this plan? Is this feasible?
2. If yes, what are the pros and cons of this?
3. If I let the money remain in Pre-tax, how does it impact me?

[dmertz · Accepted solution]

The employer is correct that if they were not the ones who made the mistake and you have not made an excess contribution, they are not permitted to reclassify the contribution.

Unfortunately, your proposed plan will not work because the excess would be deemed to be in the designated Roth account.  If you request the return of an excess contribution, the employer will make the corrective distribution from the designated Roth account.

If you have pre-tax funds in the 401(k) at the previous employer, perhaps from employer matching contributions, you can do a taxable rollover of some or all of that to a Roth IRA.

If the current plan allows In-plan Roth Rollovers, simply do an in-plan Roth Rollover of the amount unintentionally contributed as  pre-tax.

If you do nothing, you'll simply have an amount that will be excluded from income now (excluded from box 1 of your W-2) and will taxable later when distributed and not rolled over to another pre-tax account.  If the current employer makes pre-tax matching or profit sharing contributions, these pre-tax amounts will just be combined in the pre-tax account.