If you pay for your drugs or doctor visit and you didn’t have your HSA checkbook with you, or your HSA debit card in your purse, pay with your “normal” taxable money. Then, after the fact, write yourself a check to reimburse yourself for the qualified medical expense.
Another typical event is the following: You haven’t even set up your HSA savings account, but you need to spend money for medical expenses. Just keep your receipts. When you open up your savings account, you can write yourself a check for those expenses. In fact, you have until April 15th of the following year to open up an HSA Savings account and still have it affect your deductions for the current year.