People can be hurt physically when a normally calm person suddenly strikes out, and they do not understand what is happening. That person could be experiencing a psychotic break with reality, and there are a variety of causes. Each person will have to be diagnosed and treated individually, but the event that triggers it is often dangerous to those around the patient.
Dealing with a person when reality is unreal to them can be a situation where many people will be hurt. The lack of understanding that comes with this mental health condition begins with the fact it is unseen, and there is often no way to predict it will occur. When a person experiences a sudden break with the world around them, their actions are often defensive to them because they believe they are being attacked, and they have no understanding they are harming innocent bystanders. The trigger could be something simple that causes them to lose touch with reality, or it could be a complex issue that has been building for years. Professional mental health treatment over time may be the only assistance that will help.