Consider building a calendar app. You build part of the thing, but you have not handled error conditions and edge cases, the back-end supports only a single user and you don't have authentication working.

It's enough to validate some of the flows and how UX actually works, but if you show it to a customer with the actual proper designs they will subconsciously asume that the work is finished.

If the look and feel is "this is not real" it sets their mind in a different configuration.