The Biggest Retirement Planning Mistakes I Saw as a CFP

Mistake 1: The Spreadsheet That Assumed 7% Every Year

I reviewed client-built retirement projections regularly. The most common flaw: a single assumed return — usually 7% — applied uniformly every year.

The problem isn't the 7% (that's a reasonable long-run equity assumption). The problem is that retirement outcomes aren't determined by average returns — they're determined by the sequence of returns and the interaction between withdrawals and volatility.

A 7% straight-line projection shows your portfolio growing smoothly. A Monte Carlo simulation shows that in roughly 10-15% of simulated futures, the market gives you three consecutive bad years in your first decade of retirement — and your portfolio never recovers. The straight-line model doesn't show that because it doesn't exist in the straight-line world.

Every client who brought me a spreadsheet with a single return assumption was surprised when the Monte Carlo analysis showed lower success rates than the spreadsheet projected. The spreadsheet isn't wrong — it's just measuring a world that doesn't exist.

Mistake 2: Not Modeling the Surviving Spouse Separately

Married couples often plan as a unit and never model what happens to the survivor.

Here's what actually happens when the first spouse dies: - Social Security: the household loses the smaller of the two Social Security checks. A couple receiving $3,400 + $1,800 = $5,200/month becomes a single survivor receiving $3,400/month. - Pension: If the pension was selected as single-life, the survivor receives nothing. Joint survivor pensions pay a reduced benefit (typically 50-75%). - Taxes: The surviving spouse now files as Single — narrower brackets, lower standard deduction. The same Social Security income is taxed more heavily. - Housing: If the couple owned a home together and the survivor wants to remain, they now manage the property on a single budget.

The standard financial plan shows the joint scenario. The surviving-spouse scenario is the stress test that tells you whether the plan actually works for the person who outlives the other — statistically, a woman has a median survival age 3-5 years longer than her husband. This is not a morbid exercise. It's planning for the person who is most likely to live the longest.

Mistake 3: Underestimating Inflation on Spending

Retirement projections often use a single inflation rate (usually 2-3%) applied uniformly. The problem: specific categories of retirement spending inflate much faster.

Historical healthcare inflation: approximately 5-6% per year. At 5.5% inflation, healthcare costs double every 13 years. A couple spending $12,000/year on healthcare at 65 is spending $24,000/year at 78 and $48,000/year at 91 — in nominal terms.

Property taxes and homeowner's insurance have inflated at 4-5% annually in many markets over the past decade.

At the same time, travel and discretionary spending often declines with age — the 'go-go' years naturally shift into the 'slow-go' years. A blended inflation rate that applies uniform CPI to all spending misses both the healthcare acceleration and the discretionary deceleration.

The more accurate model: use age-adjusted spending curves with category-specific inflation. This produces a more realistic (and often more favorable) picture of long-term sustainability once you account for both the healthcare headwind and the natural activity decline.

Mistake 4: Treating Social Security Claiming in Isolation

Social Security claiming decisions are not standalone — they interact with everything else in the plan.

The most common isolation error: deciding when to claim Social Security without modeling the Roth conversion opportunity it creates.

A married couple where both spouses delay Social Security to 70 might have ages 60-70 where their income is solely from portfolio distributions — potentially a 10-year window where their tax bracket is the lowest it will be for the rest of their lives. This is the optimal window for Roth conversions.

If instead they claim at 62, their Social Security income begins immediately, occupying the lower tax brackets and potentially reducing Roth conversion capacity. The early claim might feel 'free' (it produces income!), but it may cost $50,000-$150,000 in lifetime taxes by foreclosing the conversion window.

The analysis that reveals this: run the plan with delayed Social Security + aggressive Roth conversions versus early claiming without conversions. The lifetime after-tax outcome comparison often shows six-figure differences.