Prototype of a software to avoid uncertainties

We make a prototype of a software to avoid uncertainties, Please list out some type of uncertainty which we may have to face?

Prototype is used when there is uncertainty regarding requirements. Sometimes, even the customer does not know what he/she actually needs. In this case, prototype is helpful. We enable user to have an idea about final product. He/she can identify if there is any missing requirements.
Uncertain situations can be:

User is unaware of the all requirements
User is not technical enough to give required details about requirements.
Requirements are creeping
Project scope is creeping etc.