I spent some time bisecting the SVN history in the py3k branch, and believe that r84914 is the commit that introduced this issue.

Details:
  
  Trying on 4-core i386 RHEL 5 box
  $ svn up -r REV
  $ make clean ; make
  (configured --with-pydebug)

Reproducer:

  ./python -c "import multiprocessing.managers ; mpp = multiprocessing.Pool(4); sm = multiprocessing.managers.SyncManager(); sm.start()"

  (running it 3 times; each time, at each build I see it either successfully completing 3 times, or having the Py_FatalError 3 times)

Bisecting:
  r86720: CRASHES with r86729 --with-pydebug (from when I was investigating this before)
  r73573: Runs OK with r73573 --with-pydebug (r73573 was origin for 3.1 tag)

    => somewhere in 73572..86729

  r76195: Runs OK with r76195 --with-pydebug (r76195 added the new GIL)

    => somewhere in 76195..86729

  r80724: Runs OK with r80724 --with-pydebug (changed threading code to "Make (most of) Python's tests pass under Thread Sanitizer.")

    => somewhere in 80724..86729

  r83722: Runs OK with r80724 --with-pydebug (touched multiprocessing)

    => somewhere in 83722..86729

  r85222: CRASHES with r85222 --with-pydebug (rough midpoint)

    => somewhere in 83722..85222

  r84472: Runs OK with r84472 --with-pydebug (arbitrary midpoint)

    => somewhere in 84472..85222

  r84847: Runs OK with r84847 --with-pydebug (arbitrary midpoint)

     => somewhere in 84847..85222

  r85033: CRASHES with r85033 --with-pydebug (arbitrary midpoint)

     => somewhere in 84847..85033

  r84938: CRASHES with r84938 --with-pydebug (arbitrary midpoint)

     => somewhere in 84847..84938

  r84892: Runs OK with r84892 --with-pydebug (arbitrary midpoint)

     => somewhere in 84892..84938

  r84915: CRASHES with r84915 --with-pydebug (arbitrary midpoint)

     => somewhere in 84892..84915

  r84903: Runs OK with r84903 --with-pydebug (arbitrary midpoint)

     => somewhere in 84903..84915

  r84914: CRASHES with r84914 --with-pydebug (affects threads)

     => somewhere in 84903..84914

  r84913: Runs OK with r84913 --with-pydebug (previous commit before 84914)

     => r84914 is the commit that triggered this issue