I love the secret questions.
I love knowing when they are coming.
When someone asks you something that is not related to the conversation at hand, is completely off-topic and is geared towards ascertaining one and only one thing.
Do you know your stuff?
The problem with the secret question is that the candidate knows it for what it is – everything you’ve spent the last hour or so talking about is fluff and this is really what you want and/or need. At that moment they will decide how they will answer – with a question, with the answer or simply excuse themselves from the answer.
If it’s the final of the three, you’ve most likely lost the candidate, not because they don’t know, but because they don’t like being astray and having the next step come down to a piece of trivia.