After running dozens of CoastFIRE scenarios for friends, coworkers, and readers, I've noticed the same mistakes come up again and again. Most of them aren't about bad math. They're about hidden assumptions that quietly make the plan fragile. Here are the five that trip people up the most, and how to fix each one.
This is the single most common error. People see "the stock market averages 10%" and plug 10% into their calculator. Then they wonder why their actual portfolio feels like it's lagging.
The truth: 10% is the nominal return. Inflation quietly erodes about 3% per year. Your real return, the one that actually tells you what you can buy, is closer to 7%.
A dollar in a Roth IRA is not the same as a dollar in a taxable brokerage. When you withdraw from the brokerage, you owe long-term capital gains (typically 15%) on the growth. Roth dollars come out tax-free. Traditional 401(k) dollars come out as ordinary income.
If you have $500K in a brokerage, you might think you have $500K. But after taxes, the real spendable amount might be closer to $440K–460K. That's a meaningful difference when you're planning a 30-year retirement.
People look at their SSA statement, see "$2,800/month at full retirement age," and plug that in. Then they forget two things:
Then there's the funding question, and it's no longer a vague worry. The 2026 Trustees Report projects the retirement trust fund (OASI) runs out of reserves in the fourth quarter of 2032. That doesn't mean benefits stop. Incoming payroll taxes would still cover about 78% of scheduled benefits. On a combined basis including disability, depletion lands in the third quarter of 2034.
Congress has always acted before a deadline like this, and will probably act again. But "probably" is doing real work in that sentence, and the fix is almost certainly some mix of higher taxes, a higher retirement age, and reduced benefits for higher earners. Building your plan on 100% of the projected benefit means betting that the repair costs you nothing.
CoastFIRE math assumes your portfolio grows at an average rate. But markets don't deliver averages. They deliver a sequence. A bad first decade after hitting CoastFIRE can be catastrophic, even if the long-term average works out.
If you hit your number at 40 and the market drops 30% in your first year, your "coast" portfolio is now 30% smaller, and compounding is working against you for 25 years. The average return might still arrive, just not in time to help you.
This is the psychological trap. You hit your CoastFIRE number and immediately think "I'm done saving!", so you stop contributing and redirect everything to lifestyle upgrades.
The problem: CoastFIRE assumes you coast for decades without adding money. If markets underperform in the first few years, you have no buffer. A $50K cushion on top of your CoastFIRE number can turn a 70% success rate into a 90% success rate.
Your CoastFIRE number isn't static. Your expenses change. Your goals change. Market conditions change. Someone who calculated in 2020 with 2% inflation assumptions needed to recalculate in 2022 with 8% inflation.
Rerun your numbers at least once a year, and anytime there's a major life change: new job, marriage, kids, home purchase, layoff. The number drifts. Your plan should drift with it.
Rerun Your CoastFIRE Numbers →None of these mistakes are fatal on their own. Make all five and your plan is brittle. Avoid them and CoastFIRE actually delivers on its promise: a concrete, reachable milestone that gives you freedom without fantasy.
If you've already calculated your number, go rerun it with honest inputs. Real returns. Tax-adjusted balances. Conservative SS. You might find you're further along than you think, or you might find you need a little more buffer. Either way, better to know.