People can be hesitant to do that, which can be telling of a couple things. They might not want to say because it reveals too much about their business in some way. Or perhaps because they're bashful about their code. You'll be able to tell by the interviewers behavior I'm sure.
I was asked a really great, detailed, squirrely debugging/diagnosis question a few jobs ago. After a really entertaining 10-15 minutes or so of asking follow up questions to try and solve the problem, I diagnosed the problem and provided the sequence of steps required to solve the problem. Then it suddenly struck me what was going on. It was easily as much fun as I've had on an interview in a very long time.