Here is an entry from my nightly analysis report:

New JDI_REGRESSION failures (from 2007.08.23)
    com/sun/jdi/ExceptionEvents.java
        This test failed the following assertion:

            Internal Error (src/share/vm/runtime/synchronizer.cpp:3432)
            Error: assert(false,"Non-balanced monitor enter/exit!")

        on Solaris SPARC Client VM (machine nanonano). I found the
        following bug that names this assertion:

            6302798 2/2 Java2Demo assertion failure with biased locking

        However, this bug was fixed way back in Mustang-B46.

        ###@###.### sent an e-mail reporting a sighting
        of this assertion on 2006.07.31. There was reply from Ken
        asking for more details, but I have no further e-mails.

The above entry was with Dolphin-B18 JDK bits, but with a newer VM
so this is a Dolphin-B19ish failure.