Would you really expect a programmer you're hiring to know this? Is this useful knowledge?
* 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.
It's a very tellin question if they claim Java experience and can't say anything coherent about leaks or "unintentionally reachable" objects.
It means both that they haven't had to attack memory pressure issues, haven't used heap dump tools, etc., and haven't had the intellectual curiosity to know how the platform works.
The question as-is ("How do you create a memory leak in java?") is poorly constructed and will, a significant portion of time, lead down the wrong path when interviewing. Both you and the interviewee have to make assumptions.
If you want to know if they have had to deal optimizing memory usage, simply ask. What was the problem? How did you detect it? How did you solve it? No ambiguity, no assumptions.
"It's a very tellin question if they claim Java experience and can't say anything coherent about leaks or "unintentionally reachable" objects."
You didn't answer the root of the OPs question -- why is that knowledge important to the application you are building or the work you are doing? Sorry, you failed his interview...