If +PrintClassHistogram is used with a fastdebug/debug build and sharing is enabled when the it triggers an assert in KlassInfoTable::hash (src/share/vm/memory/heapInspection.cpp). Here's the head of a sample error log:

#
# An unexpected error has been detected by Java Runtime Environment:
#
# Internal Error (/net/prt-solx86-q1-2/PrtBuildDir/workspace/src/share/vm/memory/heapInspection.cpp, 111), pid=16343, tid=5
#
# Java VM: Java HotSpot(TM) Client VM (20060112160512.dcubed.service_hs_merge-debug mixed mode, sharing)
#
# Error: assert(_pmr.contains((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
#

From a brief glance it appear the assert checks that the given klassOop is in mem region for the perm generation. When sharing is enabled the perm gen has multiple spaces (the space, ro sparc, rw space) and the assert only checks that the klassOop is in "the space".

This assertion can occur when the following regression test is run:
  sun/tools/jmap/Basic.sh

as this test will trigger a heap inspection.