Types of error.
You decide to forgo one chapter and concentrate on the rest but the question turn out to be easy. However, if you have spread your time to all chapters, you would be able to do all of them. The cost of this mistake is small.

You thought that a chapter is tested but you found out that there is a email saying that it is actually not tested. The cost of the mistake is higher because it is very unlikely to miss the email. However for the first mistake it is not very frustrating because it is hard to guess how the question will come out in what fashion.

Common reaction:
The more costly or bad a decision, incident or error is, people will be more emotional, more shock, more frustrated.

The less frequent or less likely something an incident, wrong decision or error will happen, the more effect it will have on people if it happens.

Ideal case:
We plan to be calm no matter how minor or serious a decision, error or incident because being nervous, frustrated or emotional is suffering.

Correct attitude:
Rare, very costly or very big mistake or unlucky incidents do happen in life so we must accept this fact.

Ways to practice calm in this type of shock:
Search for examples and classify them, imagine them happening and learn from them.