Leave Room for Error

"The most important part of every plan is planning on your plan not going according to plan." — Morgan Housel, The Psychology of Money (2020)

In The Psychology of Money, Morgan Housel devotes a chapter to an idea engineers and pilots take for granted but savers often ignore: room for error, the margin of safety that lets you survive being wrong. The future is uncertain, so the sensible response is not a sharper forecast but a bigger buffer.

Applied to your money, this is the principle behind every cushion in this pillar. You hold more cash than you expect to need, because you cannot know which month the shock arrives. You assume your income might dip and your costs might rise, and you build a plan that still works if both happen at once. The goal is endurance, not precision.

Housel's deeper point is that room for error is what keeps you in the game long enough for the good things — compounding, a recovering market, a career that builds — to actually happen. The investor or household wiped out by a single bad surprise never gets to enjoy the long run. Survival is the precondition for everything else.

So when you size a buffer, lean generous. When you plan a budget, assume some things go wrong. The cost of a slightly-too-large margin is a little lost yield; the cost of too small a margin can be ruin. That asymmetry is the whole argument.

Illustrative example: planning to survive the bad case

The chart shows a spread of possible outcomes around what you expect. Room for error means building your plan to withstand the shaded downside — the worse-than-expected cases — not just the comfortable middle. Plan for the bad tail, and the good cases take care of themselves.