A good unit test doesn't mirror the function it is testing.

As a greatly simplified example, consider you have a function that returns an average of two int's. The most comprehensive test would call the function and check if a result is in fact an average. This doesn't make any sense at all: you are mirroring (replicating) the functionality you are testing. If you made a mistake in the main function, you will make the same mistake in the test.

In other words, if you find yourself replicating the main functionality in the unit test, it is most likely a sign that you are wasting your time.

A good unit test doesn't mirror the function it is testing.

As a greatly simplified example, consider you have a function that returns an average of two int's. The most comprehensive test would call the function and check if a result is in fact an average. This doesn't make any sense at all: you are mirroring (replicating) the functionality you are testing. If you made a mistake in the main function, you will make the same mistake in the test.

In other words, if you find yourself replicating the main functionality in the unit test, it's a likely sign that you are wasting your time.