This is a restatement of the old wisdom that to safely use a tool you must be 10% smarter than it is." Or stated differently, you must be "ahead" of the tool (capable of accurately modeling and predicting the outcome), not "behind" (only reacting). TDD is kind of an outgrowth of it. I've lived by the wisdom, but admit that for me there is a lot of fun in the act of verifying hypotheses in the course of development, even in the "test case gap" when you're writing the lines of code that don't make a difference in terms of making a long term test case go from red to green, or doing other exploratory work where the totality of behavior is not well charted. Those times are the best. "Moodily scowling at the computer screen again," has been a status update from chilluns on what I'm doing more times than I like to admit.