refcount leak warnings

I have a PL/Java user that performs some lengthy operations. Eventually,
he get warnings like:

WARNING: buffer refcount leak: [779] (rel=1663/16440/52989, blockNum=3,
flags=0x27, refcount=1 2)

I traced this to the function PrintBufferLeakWarning. AFAICS, it's only
called from the function ResourceOwnerReleaseInternal under the
following comment:

* During a commit, there shouldn't be any remaining pins --- that
* would indicate failure to clean up the executor correctly --- so
* issue warnings. In the abort case, just clean up quietly.

I have no idea where to go from here. What should I look for when trying
to find the cause of such warnings?

Regards,
Thomas Hallgren