I've been helping my parents navigate the world of estimated tax payments while they complete rollovers from traditional IRAs to Roth IRAs in their retirement. We've been doing good so far, relying on prior year filings to calculate the equal quarterly installments that are required. Until now. For some reason that I have yet to learn, my dad sent *more* than he needed to in this month's payment (to both the US Treasure and to our state's revenue department). My guess is that he got confused with what the past estimated payments were and sent that amount. So, I'm guessing that mom and dad will end up getting that back in the form of higher refunds from both the state and Feds next year. But is that the end of the story? Or will the taxing authorities somehow believe that the smaller (yet correct) payments were somehow underpayments and force a penalty or interest? Do I need to help them take any measures to explain to taxing authorities how it was a mix-up to their benefit, not my parents' benefit, etc.?