While investigating a G1 issue with a customer supplied test case I tried running the test case with a 32 bit client JVM on our batoka machine. The original issue is a possible race with concurrent marking so I ran the test case with ParallelMarkingThreads=1.
After around 4 hours the test case failed with an assert that the contents of an SATB buffer was not an oop.
Examining the resulting core file the value that was being triggering the assert was 0xd which may possibly be a mark word of an oop.