Estimated taxes and not taxes actually owed? Such as you'd pay with a 1040-ES form? Then you don't have a problem. You can make the additional payment now if it's a large amount, or add it to the next payment if it's a small amount. Just use another coupon and mark it with the same quarter/year.
Estimated taxes are an assumption of what you're going to earn, and how much the IRS expects you to pay through the year on it. Worst case scenario is an underpayment penalty later if the amount is big enough and you don't pay it.