The 401(k) Mistake That Quietly Costs You Six Figures

Your 401(k) might be losing you money right now.

Not in a crash. In the settings you set once and never looked at again.

That quiet leak, left alone for a career, can cost you six figures.

The 401(k) is the best wealth-building tool most people will ever touch. It is also the one they pay the least attention to. You sign up on your first day at a job, pick a number in a hurry, check a box, and never open it again. Meanwhile the small mistakes baked into that first day compound for thirty years, right alongside your money.

The good news: almost every one of them is fixable in a single afternoon. Here are the five that cost the most.

Before the list, one thing about the phrase “six figures,” because it sounds like hype. It isn’t. A few thousand dollars a year, or a single balance left to rot, does not stay small. At a normal market return, money roughly doubles every decade. So a mistake that costs you $4,000 a year in your thirties is not a $4,000 mistake. Stretched across a career, it is a six-figure one. That is the quiet math running underneath every point below.

Mistake 1: Leaving the free money on the table

Start with the most expensive one, because it is pure, instant loss.

Most employers match part of what you contribute. A common formula is fifty cents on the dollar up to 6% of your pay, and plenty of companies match dollar for dollar. The average match lands somewhere around 4 to 6% of salary. Whatever the exact number, this is the rarest thing in all of investing: a guaranteed, immediate return of 50% or 100% on your money, before it has grown a cent.

If you contribute less than the amount your employer matches, you are turning down a raise. On a $70,000 salary with a full match up to 6%, skipping it leaves roughly $4,200 a year on the table. Invested over a career, that alone is a six-figure hole.

The fix takes ten minutes: log in, find your contribution rate, and set it to at least capture the entire match. That is the single highest-return financial move available to almost anyone.

Mistake 2: Abandoning your old 401(k)s

Here is a problem that has quietly exploded.

Americans now have roughly $2.1 trillion sitting in about 31.9 million forgotten 401(k) accounts, left behind at old employers. The average abandoned account holds around $66,000. People change jobs, mean to deal with the old plan later, and never do. Later becomes never.

A forgotten account is not just untidy. It is often stranded in whatever default fund it landed in years ago, sometimes a cash-like option earning almost nothing, quietly bleeding value to fees while you forget it exists. In a worst case, a single left-behind account can cost you more than $500,000 in foregone growth over thirty years.

The fix: track down every old plan and consolidate. Start by listing every employer you have ever contributed through, then contact each plan administrator, and check the national registry of unclaimed retirement benefits for anything you have lost track of. Roll what you find into your current employer’s plan or into an IRA, where you can see everything in one place and actually control how it is invested. One afternoon of paperwork can rescue an account you had written off.

Mistake 3: Letting fees quietly eat your returns

This is the mistake nobody feels, because it never shows up as a charge you notice.

Every fund inside your 401(k) has an expense ratio, a percentage skimmed off every year. It sounds trivial. It is not. A difference of just 1% in annual fees can shrink your final balance by nearly 28% over a career. In real numbers, two savers with identical contributions over 40 years can end up more than $100,000 apart based on fees alone.

As John Bogle, who built the first index fund, put it:

In investing, you get what you don’t pay for.

The good news is that low-cost options have become the norm. The average expense ratio on index and target-date funds has fallen to roughly 0.25%, but plenty of plans still park people in funds charging four or five times that. The fix: open your plan, look up the expense ratio on each fund you own, and move to the lowest-cost broad index or target-date fund available. Aim for well under 0.30%. You will likely never notice the change day to day, and it can be worth a six-figure difference by the end.

Mistake 4: Never leaving the default contribution rate

Auto-enrollment did millions of people a favor by signing them up automatically. Then it quietly trapped them.

Many plans auto-enroll you at a low default rate, often 3%. That is enough to feel like you are saving, and far too little to actually retire well. Worse, it often sits below the full employer match, which drops you straight back into Mistake 1. People assume the default was chosen for their benefit. It wasn’t. It was chosen to be painless.

For context, the 2026 limit lets you contribute up to $24,500 of your own money, plus another $8,000 if you are 50 or older. You do not need to jump there overnight. But 3% is a starting line, not a destination.

The fix: raise your contribution by 1 to 2% today, then turn on auto-escalation so it climbs automatically each year. You adjust to the small increases almost immediately, and the difference between saving 3% and 15% over a career is not thousands of dollars. It is hundreds of thousands.

Mistake 5: Cashing out when you change jobs

This is the most tempting mistake, and the most destructive.

When you leave a job, that 401(k) balance suddenly looks like available cash. A huge number of people, especially with smaller balances, simply cash it out. The damage is triple: you pay income tax on the whole amount, you add a 10% early-withdrawal penalty if you are under 59½, and, worst of all, you kill decades of future compounding on money that can never be replaced.

As Charlie Munger said:

The first rule of compounding: Never interrupt it unnecessarily.

Cashing out an old 401(k) is the definition of interrupting it. A $30,000 balance cashed out in your thirties is not a $30,000 mistake. It is the $300,000 or more it would have become, gone.

The fix is simply to not do it. When you leave a job, roll the balance into your new plan or an IRA. Keep the compounding running, untouched.

Your move this week

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Notice that none of these five is about picking hot investments or timing the market. They are about not quietly bleeding value through neglect. That is what makes them so fixable, and so worth fixing.

So block one afternoon this week and run the checklist. Capture the full match. Track down and consolidate old accounts. Check your fees. Nudge up your contribution and turn on auto-escalation. And promise yourself you will never cash out an old plan again.

None of it is complicated. It is just the maintenance almost nobody does, which is exactly why doing it puts you so far ahead.

Which of these five is costing you the most right now? Tell me in the responses, and follow along for more on building wealth without the guesswork.

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