Most retirees don't overpay taxes by accident. They overpay by default.

The situation. A couple retired at 63 with $1.1M in traditional IRAs. Between ages 63 and 72, they lived on cash and Social Security, keeping their taxable income low.

The mistake. They didn't do a single Roth conversion in those low-income years. Their CPA never brought it up. They didn't know to ask.

The outcome. At 73, RMDs kicked in at nearly $60,000/year and climbed from there. They were suddenly in the 22% bracket, watching more of their Social Security get taxed, and paying IRMAA surcharges on Medicare. Nine years of the lowest-tax years of their life — gone.

The situation. A retiree started taking Social Security at 66 and pulled $50,000/year from his 401(k) on top.

The mistake. He assumed Social Security was tax-free. It's not — up to 85% of the benefit becomes taxable once your other income crosses a threshold.

The outcome. He got a $4,200 tax bill in April he hadn't budgeted for, and a Medicare IRMAA surcharge the following year on top of it. His 'safe' 4% withdrawal quietly became 4.6% after taxes.

The situation. A retiree did a large one-time Roth conversion at 71 without checking the Medicare income thresholds.

The mistake. The conversion pushed his modified adjusted gross income $2,000 over an IRMAA bracket. IRMAA is a cliff, not a slope — one dollar over and you pay the surcharge on the whole bracket.

The outcome. That $2,000 of extra income cost him and his wife an extra $2,900 in Medicare Part B and D surcharges for the entire following year. Effective marginal rate on that last $2,000: 145%.

The situation. After her husband died, a widow inherited his IRA on top of hers. Her RMD requirement roughly doubled.

The mistake. No pre-planning had been done to reduce the traditional balances before either spouse's death — no Roth conversions, no qualified charitable distributions.

The outcome. Her RMDs plus survivor Social Security put her in a higher bracket than she and her husband had ever been in together. She paid more tax as a widow than they had as a couple.