The most expensive mistake we see is not building the wrong thing. It's building the right thing so that only one person can change it.
It looks like speed at first. One engineer holds the whole picture in their head, skips the write-up, skips the tests, and ships fast. On the surface nothing is wrong: the product works, users are happy. The cost shows up later, when that person is unavailable, or busy, or gone, and a small change to how payments are attributed suddenly needs a week of reading code before you can even start. That's when founders pay twice, once for the original build and once for the rewrite.
What prevents it is boring and cheap. More than one person has read every important part of the system, there are tests that explain what the thing is supposed to do, and the setup steps live somewhere other than someone's memory. We cover roles rather than individuals for exactly this reason: if one engineer is out, the work keeps moving.
The test is simple. Someone new should be able to make a small change safely next week, without the original author in the room.