I have had this problem in real life. I had a graph database, and I needed to find the end of a path in the graph. If the the path looped back on itself, my program would not terminate.
This occurred to me as well: the graph data model is one of the few areas in which you might practically need to address this problem. Again, however, a lot of these issues specific to the graph data model are open research questions and not really appropriate to an engineering interview.
Finding the loop in a graph is a much more valid scenario and any candidate aware of traversal algorithms will have a much better chance of dealing with it than finding loop in a LL.
