It seems that if the tests are run in this order:

./python -E -tt ./Lib/test/regrtest.py -u all test_xmlrpc test_ctypes
test_json test_bsddb3 test_pickletools

The error will trigger consistently.  That is in 2.6 with a debug build
on a dual cpu box.  A debug build of 3.0 on the same machine did not
fail though I don't know if 3.0 has this problem. I was unable to prune
the list further.  The 3 tests (xmlrpc, ctypes and json) can be run in
any order prior to bsdb3 and then pickletools.