Mistake or not, the participant's actual election was executed, so I say have them fix it going forward and deal with it. Why is it always the collective "we" - plan sponsors, advisors, TPAs, RKs - that are asked to bend over backwards to accommodate a participant's mistake, poor judgment, or lack of attention? When is the participant held accountable for not doing what (s)he is supposed to and then months or years later comes looking for help on situation (s)he could have rectified almost immediately had (s)he paid the slightest attention? I'm sorry, but if I intended to make a PRE-TAX deferral from my pay and my income tax withholdings remained the same, I would have noticed and said something - if not after the first pay period, certainly within a few.
Sorry for the rant, and I don't do this administration so I don't deal with these situations - but you all do - and don't you have enough work and have enough plan sponsor and advisor administrative "issues" to fix already?
OK, I'm done.
Also, it's 9/11, so let's remember those we lost that terrible day and from its aftermath.