If the plan itself made the error and created the excess then they MUST return the excess or the plan could be disqualified whether before or after April 15. If the excess was created because there were two or more employers (as is usually the case with excesses) and each employer did not exceed the limits then neither employer is obligated to return it., The plan is obligated to be sure that they do not over withhold and create an excess and if they do it must be returned. That would be reported to you with a 1099-R box 7 code E.
If the plan is a non-qualified plan then the rules are different but a 401(k) is a qualified plan.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
