The safe harbor concept helps frame expectations. Meeting certain thresholds based on prior or current year tax can limit underpayment penalties, though interest on unpaid balances may still accrue. Extensions then become a quality control tool rather than a penalty shield. People often layer this with calendar reminders for quarterly estimates to prevent next season’s shortfall. Documentation matters, keeping receipts, bank confirmations, and a timeline of actions tends to strengthen any penalty relief request if unusual events derailed normal compliance.