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Been there, done that, would not recommend
(feddit.org)
Its definitely a bad idea writing new code that builds up on your old code, that has not been tested properly, because you quickly have to start debugging multiple layer is code at once.
From what I understand the difference between how I code and what has Ben described as TDD in this thread is, that I set up everything first and then try to think of ways people could break the code and then test these vases/try to prevent them.