We should program for change AKA the Open/Closed Principle. In my opinion, the OCP is one of the lesser respected SOLID principles. One of my biggest, and earliest failures fresh out of university was ignoring this concept.
At the time I was applying YAGNI to some code myself and a couple of other developers were working on. After all agile methodologies promote this concept heavily. This made sense to me. My solution was to solve the problem with the minimal amount of fuss, however in doing so I strongly coupled the code we produced with the direct business requirements.
The requirements stated that we would have three different types expenses. So I promoted that we model these three types of expenses directly. The UI knew about these expenses. The database knew about these expenses. The domain logic knew about these expenses.
Everything worked well for a while. We finished early. We wrote just the code we needed. I was happy. Until the business requirements changed. The three types of expenses became four, then three again, then one was replaced completely. Bugger.
The code was unusable. Everything knew just enough to get by, so when the change came in, everything needed to change. My team was confident this would be OK. After a few hours of analysis, we concluded the code was a train wreck. We'd need to restart from the beginning in order to make the proper changes we wanted. I was pretty gutted, however I learned a very important lesson.
YAGNI is about features, not code.
If I was to complete this feature again, I would still start with the simplest thing that could possibly work. Most likely the code would explicitly know about each type of expense, yet my tests would be wrote in an agnostic manner. I would still apply YAGNI, but at a feature level. In other words, I wouldn't write an expense logger, if all we need to do is validate and calculate expense totals.
During each refactor stage of the TDD cycle I would remove any specific expense knowledge. After a while I would end up with the various parts of the application working with a generic expense algorithm. The tests would drive us towards how the algorithm would work.
The beauty here is that if a new expense was to be introduced, this change would be data driven. We would be able to give this the business for "free".
I still regret this mistake, but this lesson has lived with for some time and has proved to be a valuable experience.