https://buildbot.python.org/all/#/builders/73/builds/208
---
Warning -- files was modified by test_threading
  Before: []
  After:  ['core']

1 test altered the execution environment:
    test_threading
---

The bug can be reproduced with:

$ ./configure --with-pydebug CC=gcc CFLAGS=-O0
$ make
(...
$ ./python -m test test_threading --fail-env-changed -m test.test_threading.ThreadJoinOnShutdown.test_reinit_tls_after_fork  -F -j 20
0:00:00 Run tests in parallel using 20 child processes
0:00:01 [  1] test_threading passed
0:00:01 [  2] test_threading passed
(...)
0:00:03 [ 17] test_threading passed
0:00:03 [ 18/1] test_threading failed (env changed)
Warning -- files was modified by test_threading
  Before: []
  After:  ['core']
Kill <TestWorkerProcess #1 running test=test_threading pid=14156142 time=1.6 sec>
(...)
Tests result: ENV CHANGED