Treat the customers with respect. Assumptions are there to be challenged. If you had experienced where a particular feature failed, use it to back you up. Other

Treat the customers with respect. Assumptions are there to be challenged. If you had experienced where a particular feature failed, use it to back you up. Otherwise, acknowledge their request, list it down as a point where the two of you disagree. Then suggest that it would be interesting to challenge your own assumptions and experience in some areas, to see if some of these have changed. But to run good tests, it's best to limit the number of changes, or to limit the risks to the customer.