A mass delusion is a spontaneous spreading of false beliefs and fits into Sherif’s study of norm formation.
One such example of this is the Salem Witch Trials. As we all remember, probably, the trials were held because eight girls were acting strangely and some people decided that was because they were witches. Many died because of these trials.
The Salem Witch Trials are an example of mass delusion because after the girls were pronounced witches, randomly other people were seen as witches and then executed. As can be seen, a spontaneous belief about the existence of witches occurred (which is a false belief), thus exemplifying the definition of mass delusion.