Common Investing Mistakes and How to Undo Them

Strategy & FAQ · October 11, 2026

Almost nobody destroys a portfolio with a single spectacular decision. The damage is usually accumulative: a fund bought after a good run, a balance left in cash for two years, a fee nobody checked, a sale made during a frightening week. Each one looks minor at the time. Together they explain why so many people hold accounts that have been busy for a decade without getting very far.

The encouraging part is that these are habits, not personality traits. Habits can be reversed in an afternoon, usually with a form rather than a forecast.

Buying what already went up

A fund that returned far more than the market last year gets a lot of attention, and money tends to arrive after the good news rather than before it. The result is a portfolio of yesterday's winners purchased at yesterday's high prices. The undoing is mechanical: stop choosing holdings by recent performance and choose by a written allocation. If you already own a stack of hot funds, list their overlap before selling anything; you may find you own the same few companies five times.

Leaving contributions in cash

Funding an account and never buying anything is more common than people admit. It feels cautious, but over years it quietly converts a long-horizon plan into a savings account with extra steps. Log in, find the uninvested balance, and set up the automatic purchase. If the reason for hesitating is that the market looks high, that is a timing view you should write down rather than leave implicit in a cash balance.

Paying fees you never agreed to

Expense ratios, advisory charges, account fees, and fund transaction costs are easy to ignore because no bill ever arrives. Open the annual statement and add them up. A total near a percent a year is a serious hurdle that compounds against you. Replacing an expensive holding with a broad low-cost one is usually a single trade, and in a tax-advantaged account it typically has no immediate tax consequence.

A crumpled sheet of paper in a wicker basket beside a tidy desk

Selling because of a headline

Markets fall, headlines explain why, and the explanation always sounds convincing in retrospect. Selling during that week turns a temporary price change into a permanent decision, and it creates a second problem: when to buy back. Most people wait for calm, which arrives only after prices have recovered. The antidote is a rule written in advance, such as rebalancing on a fixed date and never selling purely because a price moved. Put it in a note where you will see it.

Checking too often

Frequent checking does not add information to a twenty-year plan; it adds opportunities to interfere. A quarterly glance and an annual review are enough. If the app is on your home screen, move it. This is not a test of discipline so much as a design choice, and design beats willpower.

Owning too many overlapping funds

Ten funds can feel diversified while concentrating the same handful of large companies across all of them. Count the top holdings rather than the number of tickers. Consolidating into two or three broad holdings usually lowers cost, makes the allocation legible, and reduces the number of decisions you have to revisit. In a taxable account, check the gain before selling; in a retirement account, the cleanup is simpler.

Never revisiting the beneficiary and the emergency fund

These are not investment decisions, which is exactly why they get skipped. An outdated beneficiary form can override what you intended, and a thin cash reserve can force you to sell investments at the worst possible moment. Both take a few minutes to check and have consequences far larger than the effort suggests.

How to actually undo them

Print the account statements and write down what you own, what it costs, and what the cash balance is.
Write a one-paragraph target mix and a date for your annual review.
Fix the automatic contribution and the automatic purchase in the same session.
Clean up one thing at a time: cash first, then fees, then overlap, then beneficiaries.
Leave the rest alone for a year.

None of these steps requires a view on where the market goes next. That is their virtue. The point of a repair session is not to find a clever new position; it is to remove the small frictions that have been quietly taking the returns you already earned.

If you recognise three or four of the habits above, there is no need to feel foolish. Nearly every long-term investor has made most of them. The difference is usually just the afternoon someone finally spent cleaning them up.