It depends on how you define brittle.

Functional code that is perhaps overly defensive or specific in ways that will require updating it in the future? All the time.

What many do not yet understand is that the maintenance burden is a fraction of the effort that it takes you to review the code, much less iterate on it.

I see people on here talk about a supposed eventual collapse of maintainability if they do not review or understand the code.

In reality this never happens. The latest models can salvage and refactor even the biggest mess; all you need to do is ask.

They could probably do it even if source code isn't available and you only have a binary to work with.