I can relate. An older version of the Eclipse debugger would run into a stack overflow when the debugged process had a stack overflow and you paused the affected thread. It took me a while to figure that one...
Not the hardest bug I've had, but working in Common Lisp I've run into macros that crash the runtime (SBCL) when you attempt to see what they expand into. Makes it difficult to debug them.
similar here. bug in jvm ref counting. not only did the java have to be debugged, the jvm itself then had to be debugged. second hardest bug i ever had to deal with.
What's so hard about that? Just load the core dump into a debugger! Well loading the core dump into the debugger caused the debugger to dump core...