You use other rules of thumb than I do, obviously.

Of course it is (very) important to know your input data before/when you start writing code to read/parse it, but on the other hand, I propagate "defensive programming". The OT never stated that the data example is complete. That implies that you might be right today, but you might be wrong tomorrow.

I usually follow my rule of thumb: implements the minimal requirements but expect the worst. What you want to do when something is not strictly matching the original example is completely up to the author of the code, but my experience in data munging over the past 25 years is that the data format WILL change.