Seven Expensive Investing Mistakes (and the Numbers Behind Them)

The classic and the costliest: selling into a crash converts a temporary drawdown into a permanent loss, and re-entry usually happens well above the exit (see the timing guide's missed-best-days arithmetic). An investor who sold a balanced portfolio near the 2009 or 2020 lows and waited for "clarity" could easily have missed rebounds worth 30% to 60%.

The fix is structural, not motivational: hold an allocation whose historical worst case you have SEEN and accepted (stress test it), so the bad year arrives pre-survived.

Money famously floods into funds, sectors and themes AFTER their best years, which is systematically the wrong direction: winners mean-revert far more often than they repeat. Buying whatever tripled last year is how investors bought tech in 2000 and crypto in 2021: at the top of the chart they were admiring.

The antidote is owning everything cheaply (the index catches the next winner by construction) and letting scheduled rebalancing do the buy-low, sell-high mechanically.

An avoidable 1% annual fee consumes very roughly a sixth of a 30-year outcome (the fees guide walks the math). It is the one mistake on this list with a guaranteed cost and a five-minute fix: read the expense ratios you own and price them in dollars.

Employer stock accumulating through RSUs, one lucky winner outgrowing the portfolio, three funds that secretly hold the same mega-caps: concentration usually happens passively. One stock at 40% of net worth taking an unremarkable 60% single-stock drawdown removes about a quarter of everything (the concentration guide runs the numbers).

Measure it: the correlation tool shows what actually moves together, including positions that look diversified and are not.

Cash awaiting the perfect entry has historically cost more than the corrections it feared: markets spend most of their time at or near highs, and the expected cost of a year on the sidelines exceeds the expected benefit of dodging the average dip. The lump-sum-vs-DCA guide covers the honest middle path when investing a windfall feels impossible.

Comparing a diversified portfolio against a hot index (or a price-only quote) manufactures fake failure and prompts real mistakes: abandoning a sound plan to chase whatever the wrong benchmark did. Judge a portfolio against its own goals on total return, the convention every comparison in these tools uses (the total-return guide explains the trap).

Every mistake above gets easier without a plan, because every market event becomes a fresh decision made under stress. A one-page plan (target mix, contribution schedule, rebalancing rule, and what you will do in a 30% drawdown: nothing) converts future panic into procedure.