As whoppity says we are testing to see if they are safe and competent. The purpose of the test is not to demonstrate how smart the examiner is. I have only ever failed one candidate on a skill test and as Whoppity said, I could pretty much tell before the flight even started that the guy was not ready.
Our role as examiners is merely to validate the training the candidate has been given by the Instructor/School not play the smart arse.