Without a documented error on the part of the custodian, there would be no way to distinguish a mistaken designation on the part of the individual from an intentional designation for the current year and the individual later deciding (after the due date of the prior-year tax return) that they would like to have made an HSA contribution for the prior year. Allowing the contribution to be re-designated under these circumstances would effectively be allowing the individual to make a contribution for the prior year after the due date of the tax return for the prior year.
