I do a lot of interviewing in my role, and one of the first questions I ask when interviewing a candidate is
What is a Singleton?
Most candidates get it right, A class where only a single instance can be created, if the candidate does not get the right answer, I explain what it is, as not eveyone is a pattern geek, even though this is one of the most common and well known patterns.
What I ask next, is to provide a minimal implementation, in the language of choice ( most cases its one of the following, Java, C# or C++ ), now this is where it starts to get interesting. The following is an example of one recent candidates answer, they wrote this on the board with a straight face
class Singleton {
public int count;
}
This caused much discussion between the candidate and myself, obviously they did not understand anything about the C# language at all, and the concept of a private constructor was completly foreign, I then had some fun for the rest of the interview watching the candidates facial expressions as I asked more simple questions, about his implementation.
FYI
Nice Solution to the Singleton question using C#