A return of mistaken distribution does not have to be in the same calendar year as the distribution. It only has to be accomplished " no later than April 15 following the first year the account beneficiary knew or should have known the distribution was a mistake." To do a return of mistaken distribution in this case would require the HSA administrator involved to reopen the HSA, accept the return of this mistaken distribution and issue a corrected 2017 Form 1099-R. However, since an HSA is not required to accept a return of mistaken distribution, it's likely that the HSA administrator will decline to accept this one, given the circumstances.
