Consider that most "valid" implementations of fizzbuzz are wholly dependent on awareness of the target language's modulo operator. Self-taught programmers can easily overlook that, and if a specific language is requested, it's easy to not know the syntax even if the concept is understood.

Really I don't think those sort of on-the-spot tests/trivia questions are representative by themselves. They may be a useful part of a larger investigation.

Two things to stop code sample plagiarism: choose a unique code challenge and, if you're really worried about it, give the client a laptop and tell them to bang something simple out right there and come back in 30-60 minutes to check.

Honestly, though, I've rarely depended on either code samples or code trivia to determine if someone is a good hire. If you sit and talk about dev with a person, you can tell if they're on their game or not 90% of the time. The sample/on-the-spot tests can help weed out that extra 10%.

I've also heard of a story where an "Expert in C" (verbatim from resume) thought that the * in the sample source code was a typo.

I'm not sure how this is possible either, but the world works in mysterious ways.

It's possible if the person lied on their resume about actually having 10+ years of experience.