I presume this is with Memory Mapped Files or direct allocated byte buffers? I was able to leak GB as a result of this:http://developer.java.sun.com/developer/bugParade/bugs/4938372.html
I'm not convinced that the evaluation is correct --- I have also had the same problem where the memory mapped regions merely abutted rather than overlapped.
However it was fiendishly difficult to come up with the small test case that I eventually presented. Small, seemingly innocuous changes would change the behaviour.
In its current state I think the memory mapping capability is almost useless.