That's one of the reasons the JREF challenge and similar properly-organized tests insist on doing pre-testing first - to discourage people from humiliating themselves in public.
They strongly recommend that applicants do a dry run of the test protocol that's agreed before proceeding with the official test, but they can't insist.
That's why a typical application for the MDC goes:
1. Someone convinces themselves they have a paranormal ability on the basis of anecdotal evidence which is totally inadequate to support that conclusion
2. They apply for the MDC
3. JREF works with them to design an objective test protocol for their claim
4. The applicant agrees to the protocol because they can see that it is a fair test
5. The applicant doesn't bother to do a dry run of the test because they "know" that they have the ability they claim, and the test is just to convince the silly sceptics
6. The official test is run, usually with live streaming to the web.
7. The results are exactly the results that would be expected by chance; the applicant is publically humiliated
8. The applicant is bewildered and eventually decides that the test must not have been fair all along (though they can never point to anything wrong with it) and goes right on believing in their supposed paranormal ability.
See the Connie Sonne threads for a textbook example.
The only exception I'm aware of where an applicant actually did do a dry run before the official test was DowserDon, and he chose to invite two university professors to witness it so his humiliation still wasn't exactly private.