Bugs in tests exist (and are just as prevalent as bugs in production code in my experience). However, writing production code introduces a kind of constraint o

Bugs in tests exist (and are just as prevalent as bugs in production code in my experience). However, writing production code introduces a kind of constraint on the system. If you run a function with certain program state, then it returns a certain result. No matter what you do, this will happen because that's how you wrote the code. A test introduces the same constraint. Instead of implementing the function, it runs the function. No matter what you do, the test will run certain code and get a certain result. Tests pass when the constraints introduced by the production code match the constraints introduced by the tests. Because of this, if there is a software error in the production code, usually you need to have a corresponding error in the tests in order for the test to pass. This definitely happens from time to time, but the odds of it happening are much lower. Similarly, if you have a software error in the tests, you need to have a corresponding software error in the production code in order for the test to pass -- assuming that the test is actually exercising the code (something you can't always assume, unfortunately).