My situation is a bit complicated:

Worked at company A, maxed out 401k in 2017 Left company A, rolled over 401k to IRA

Started at company B. Told company B to not contribute to my 401k in their on boarding process. They did not contribute to my 401k for my paychecks to start... But, I checked my contribution for 2018 on Fidelity's site and it must have reset the contribution, because my last paycheck of 2017 there was a $2800 contribution to my 401k.

I know I can tell my employer to take it out -- but my employer has a 1/1 match (up to an amount over the $2800), so I'm wondering if it would be advantageous to pay taxes and fines on the $2800 contribution for an additional $2800 match from my employer. I don't know when the match is added, or if I'll have to pay it back if I have to distribute the $2800 contribution next year. I can't go back to my old employer's plan to ask for a distribution because, again, I rolled that over immediately to an IRA (lesson learned, do not do this with mid year job changes.)

Thoughts? I'm not interested in messing with the law here, just interested in the best way to handle this -- esp since I'm eligible for the employer match, expect I had maxed out my contributions at the earlier company.