How not to test

Testing is important - there's probably no question about that. But as soon as you go beyond the hello-world examples from the textbook, things are no longer that simple. In fact, there are a number of widespread typical pitfalls that developers like to fall into - and which ultimately mean that you have tests, but they don't work the way you actually imagined. As you gain experience, you learn to recognize and avoid these problems, but the road ahead is long and winding. Therefore, it can be helpful to learn from the mistakes of others, and to have some guidance on how to avoid the worst screw-ups, and how to build your tests properly. How does all of this work, what do you need to pay attention to, and how do you implement things correctly?