BigApp crashed with the fastdebug build on (instanceKlass() == 0 || instanceKlass()->is_instance()), and it is always reproducible in couple of minutes but only with the following VM options. I have reproduced the crash in two other machines.

VM options: "-server  -XX:+UseConcMarkSweepGC -XX:+VerifyBeforeGC -XX:+VerifyAfterGC -XX:+VerifyDuringGC"
Java version: build 1.6.0_01-b05
build 20070320194522.iv159533.gc_baseline.numa.adaptive-fastdebug

hostname: jtg-lx50-2.sfbay
logfile and core file location: /bt/atgrun.28785.-server
crash seen: tail run.atg.out
Unexpected Error
------------------------------------------------------------------------------
Internal Error at placeholders.cpp:221, pid=29188, tid=6 
Error: guarantee(instanceKlass() == 0 || instanceKlass()->is_instance(),"checking type of instanceKlass result")

How to reproduce:
export JAVA_HOME=Your_JAVA_HOME
it is set to /usr/j2se by default.

then running:
/bs/runatg.ksh <your_vm_options>

look at the results under /bt directory.
Changed App name to 'BigApp'