One big issue many face is that the code that lack tests is in fact hard to test because code is tightly coupled. So they end up with a high number of end to end tests and struggle with unit and other kinds of tests. So in addition to the points above, properly handling dependencies and designing the system are highly important as well.
Yup, this is a common pitfall for growing un-tested codebases. That's part of the beauty of TDD, that the tests you write validate your system's design 🙂
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