The costliest mistake startups make in development is building something that only one person understands.
It rarely looks like a mistake at the time. One developer moves fast because they skip whatever slows them down: no written decisions, no tests, deploys done by hand from a laptop, config living in their head. For a few months that is genuinely the cheapest way to build.
Then the bill arrives. Not as a crash, as a stall. That person goes on leave, changes jobs, or just gets busy, and suddenly nobody can tell you why payments retry twice, which environment is live, or what breaks if you touch the checkout. The code is all there and none of it is usable. We have picked up inherited systems like this (monoliths to untangle, old runtimes to migrate), and the slow part is not writing new code. It is rebuilding knowledge that nobody wrote down.
The mechanics are simple. Software costs money twice: once to build, once to keep running. The second cost is paid by whoever has to understand it next, and it compounds every month that understanding sits in one head. A codebase with tests, code review, and a documented path to production can be handed to a new engineer in days. One without those can take months to make safe to change, and you pay for those months at senior rates while shipping nothing.
This is why we work as a team with roles instead of a single pair of hands: backend, frontend, mobile, QA, DevOps, and someone owning delivery. Cross review means at least two people have read every change. If one engineer is out, the team keeps moving.
The cheap version of this fix is available to anyone, today, with no new hire: have someone other than the author deploy, and write down what they got stuck on.
And the number worth knowing is this one: if your main developer disappeared for a month, how far your product would get.