Do you have any visual example? Or textual? I'd very much like to see exactly what you mean and what would you consider a failure at that interview (and what would you consider a success).
I've asked interviewees what they could do about cycles in user-entered code, by giving them a 2-node example. Far too often, they reply with an if-clause to detect exactly a 2-node cycle. Then I ask what to do about 3 nodes, then far too many of them try to give me a 2nd if-clause!
