Mistaken distribution can be fixed (i.e. restored to the HSA).

Source quoted below:

http://www.irs.gov/instructions/i1099sa/ar02.html

HSA mistaken distributions.   If amounts were distributed during the year from an HSA because of a mistake of fact due to reasonable cause, the account beneficiary may repay the mistaken distribution no later than April 15 following the first year the account beneficiary knew or should have known the distribution was a mistake. For example, the account beneficiary reasonably, but mistakenly, believed that an expense was a qualified medical expense and was reimbursed for that expense from the HSA. The account beneficiary then repays the mistaken distribution to the HSA.

  Under these circumstances, the mistaken distribution is not included in gross income, is not subject to the additional 20% tax, and the repayment is not subject to the excise tax on excess contributions. Do not treat the repayment as a contribution on Form 5498-SA.

As the trustee or custodian, you do not have to allow beneficiaries to return a mistaken distribution to the HSA. However, if you do allow the return of the mistaken distribution, you may rely on the account beneficiary's statement that the distribution was in fact a mistake. See Notice 2004-50, 2004-33 I.R.B. 196, Q/A-76, available at www.irs.gov/irb/2004-33_IRB/ar08.html. Do not report the mistaken distribution on Form 1099-SA. Correct any filed Form 1099-SA with the IRS and the account beneficiary as soon as you become aware of the error. See Corrected Returns on Paper Forms in the 2015 General Instructions for Certain Information Returns for more information.