Learning research tells us that the time lag from experiment to feedback is critical.
If testing costs more than not testing, then don't test.
The system metaphor is a story that everyone--customers, programmers, and managers--can tell about how the system works.
Optimism is an occupational hazard of programming; feedback is the treatment.
I mean, source code in files; how quaint, how seventies!
I've known people who have not mastered their tools who are good programmers, but not a tool master who remained a mediocre programmer.