Testers don’t prevent errors, and errors aren’t necessarily waste.
Testing, in and of itself, does not prevent bugs. Platform testing that reveals a compatibility bug provides a developer with information. That information prompts him to correct an error in the product, which prevents that already-existing error from reaching and bugging a customer.
Stress testing that reveals a bug in a function provides a developer with information. That information helps her to rewrite the code and remove an error, which prevents that already-existing error from turning into a bug in an integrated build.
Review (a form of testing) that reveals an error in a specification provides a product team with information. That information helps the team in rewriting the spec correctly, which prevents that already-existing error from turning into a bug in the code.
Transpection (a form of testing) reveals an error in a designer’s idea. The conversation helps the designer to change his idea to prevent the error from turning into a design flaw.
You see? In each case, there is an error, and nothing prevented it. Just as smoke detectors don’t prevent fires, testing on its own doesn’t prevent problems. Smoke detectors direct our attention to something that’s already burning, so we can do something about it and prevent the situation from getting worse. Testing directs our attention to existing errors. Those errors will persist—presumably with consequences—unless someone makes some change that fixes them.
Some people say that errors, bugs, and problems are waste, but they are not in themselves wasteful unless no one learns from them and does something about them. On the other hand, every error that someone discovers represents an opportunity to take action that prevents the error from becoming a more serious problem. As a tester, I’m fascinated by errors. I study errors: how people commit errors (bug stories; the history of engineering), why they make errors (fallible heuristics; cognitive biases), where we we might find errors (coverage), how we might recognize errors (oracles). I love errors. Every error that is discovered represents an opportunity to learn something—and that learning can help people to change things in order to prevent future errors.
So, as a tester, I don’t prevent problems. I play a role in preventing problems by helping people to detect errors. That allows those people to prevent those errors from turning into problems that bug people.
Still squeamish about errors? Read Jerry Weinberg’s e-book, Errors: Bugs, Boo-boos, Blunders.