Act utilitarianism solely considers the consequences of the individual act, while rule utilitarianism examines the consequences of acts as a general practice of what society ought to do. To follow rule utilitarianism when a person or group evaluates a moral choice means they would choose the rule that brings the greatest happiness to the greatest number of people. For example, a doctor was under the influence of illegal drugs yesterday, although he did not cause any patients harm yesterday and he experienced a great deal of pleasure (act utilitarianism), the likely consequences (rule utilitarianism) indicate he would cause harm to one or more patients while under the influence of illegal drugs making the act ethically wrong.
To further clarify, the general principle of utilitarianism is the greatest happiness rule, and this absolute rule has no exceptions. There are specific rules people ought to follow. Some exceptions to specific rules exist, if not following the rule would bring a greater amount of happiness. Such as,