You are permitted to take a distribution from an HSA and roll that distribution back into the same HSA or a another of your HSA accounts within 60 days, subject to the limitation that you can only roll over one such distribution made in a 12-month period, the same as you can with a distribution from an IRA. If you fail to roll over the HSA distribution, either because you miss the 60-day deadline or doing so would violate the one-rollover-per-12-months limitation, and you do not apply it to qualified medical expenses, it's taxable as ordinary income and, if you are under age 65 at the time of the distribution, also subject to a 20% early-distribution penalty.
Intentionally taking a distribution that you know will not be used for qualified medical expenses would not be a mistaken distribution that you can redeposit as a return of mistaken distribution. To be eligible to be returned as a mistaken distribution you must have a good-faith belief that it is being used to pay a qualified medical expense. A mistaken distribution generally occurs when you unexpectedly receive a reimbursement of some amount already paid from the HSA for a medical expense.
