Getting the answer right isn't really important, but the candidate's answer to this question potentially reveals a lot of information.

* Most likely they've never had to think about this before. How to they approach unfamiliar territory?

* Do they have a deep enough knowledge about Java that they could at least make a conjecture about how such a thing could be created?

* Do they understand that Java has a GC and how it works?

* Are they the rigorous definition type and declare that such a thing is impossible without a bug?

* Will they try to interpret the interviewer's meaning and talk about unintended long-lived references or ways to accidentally consume a lot of memory?

Overall I like questions like this that require some creativity to answer.