This is precisely right: I ask people questions like the difference between an interface and an abstract class as a gauge, because I've had so many people answe

This is precisely right: I ask people questions like the difference between an interface and an abstract class as a gauge, because I've had so many people answer them really badly. The question wouldn't normally come out of the blue, though, but as a follow-up from part of a design exercise where they would have just used one or the other in their design, and as part of finding out whether they could justify their choice.