Offer Appropriate Encouragement

Another reason for encouraging the participant to keep trying is to show designers and developers watching the usability test the dire consequences of certain difficulties experienced by participants. It is important to let them see just how painful and frustrating the process of using their product can be.

Actually seeing the participant struggle and get very frustrated and observing the serious consequences firsthand will do more to convince a designer to  revise a product than weeks of discussions and negotiations. Sometimes just that extra amount of human struggle will convince a product designer that a change is needed and that the product should not be released as is.