[gaylemcd]

Eh... it depends.

You could have a question that can really only be solved optimally one way but that still provides adequate room to demonstrate problem-solving skills (even without knowing the "slick trick"). Such a question could still be a reasonable question.

Of course, many questions with slick tricks aren't like that. The interviewers who ask these questions also tend to be worse, which exasperates the problem.

In the situation you described - the candidate might still be considered a good candidate. It depends what the interviewer is looking for.