Pluralistic ignorance is most likely to discourage intervention in an emergency when

When the situation is ambiguous

In social psychology, pluralistic ignorance is when majority of the members of a group personally reject certain norms but go along with it because of the incorrect belief or assumption that most of the others accept such norms. It is generally a bias about a social group held by the said social group itself.

It is also though to help explain the "bystander effect" where onlookers in the vicinity will not act or intervene in an emergency because no one else is acting on the situation, leading the onlookers to believe that it might be wrong or incorrect to act in such situations. 

When you are making a guess you are forming a hypothesis