Are you sure that the $166.66 not deposited was not reflected on your 2016 W-2 in the amount in box 1 and the amount with code W in box 12? The $6,333 that is present on Form 8889 line 9 (the code W amount) is suspiciously $167 short of a nice round $6,500, but that could just be a coincidence. If your 2016 W-2 did not have the $166.66 excluded from the box 1 amount and included in the code W amount, you already paid taxes on the $166.66 when you filed your 2016 tax return. The letter is not clear on this, stating only that "income tax was not withheld from this contribution." Still, I would expect your 2016 W-2 to reflect the sum of your 2016 pay stubs.
