> ...in order to write effective tests, a programmer had to know all of the ways that a piece of software could fail in order to write tests for those cases Th

> ...in order to write effective tests, a programmer had to know all of the ways that a piece of software could fail in order to write tests for those cases That's impossible. You reason about the problem as best as you can and create tests appropriately; and then if there are edge cases/failures in production, you go back and add those to the test suite.