Comment by emmanueloga_

I'd say... anytime you are about to shove the square peg in the round hole...

More concretely, observe the complexity of nested sets [1] ... So if you need to represent trees, hierarchies or, well, graphs, in your data, maybe it could be reasonable to use a graph database instead of a relational one.

1: https://en.wikipedia.org/wiki/Nested_set_model