The following unit test is failing with fastdebug builds when the through-put collector (-XX:+UseParallelGC) is used.

    sun/tools/jmap/Basic.sh

The test exercises the heap inspection code using the jmap -histo option. With a fastdebug build and -XX:+UseParallelGC the assertion is:

#
# An unexpected error has been detected by Java Runtime Environment:
#
# Internal Error (/BUILD_AREA/jdk6.0/hotspot/src/share/vm/memory/heapInspection.cpp, 111 [ Patched ]), pid=29541, tid=5
#
# Java VM: Java HotSpot(TM) Server VM (1.6.0-rc-fastdebug-b91-debug mixed mode)
#
# Error: assert(Universe::heap()->is_in_permanent((HeapWord*)p),"all klasses in permgen")
# If you would like to submit a bug report, please visit:
# http://java.sun.com/webapps/bugreport/crash.jsp
#

So far I've been unable to duplicate this with +PrintClassHistogram.