We're seeing this error very rarely in libgraal testing (1 out of 184 executions):

Attaching to process 3042, please wait...
hsdb> hsdb> + verbose true
hsdb> + inspect 0x00000000d9488418
Error: sun.jvm.hotspot.oops.UnknownOopException: 0x00000000d9488418
sun.jvm.hotspot.oops.UnknownOopException: 0x00000000d9488418
at jdk.hotspot.agent/sun.jvm.hotspot.oops.ObjectHeap.newOop(ObjectHeap.java:193)
at jdk.hotspot.agent/sun.jvm.hotspot.CommandProcessor$25.doit(CommandProcessor.java:1109)
at jdk.hotspot.agent/sun.jvm.hotspot.CommandProcessor.executeCommand(CommandProcessor.java:2212)
at jdk.hotspot.agent/sun.jvm.hotspot.CommandProcessor.executeCommand(CommandProcessor.java:2182)
at jdk.hotspot.agent/sun.jvm.hotspot.CommandProcessor.run(CommandProcessor.java:2053)
at jdk.hotspot.agent/sun.jvm.hotspot.CLHSDB.run(CLHSDB.java:112)
at jdk.hotspot.agent/sun.jvm.hotspot.CLHSDB.main(CLHSDB.java:44)
at jdk.hotspot.agent/sun.jvm.hotspot.SALauncher.runCLHSDB(SALauncher.java:281)
at jdk.hotspot.agent/sun.jvm.hotspot.SALauncher.main(SALauncher.java:500)
hsdb> + quit