Christian Heimes reported a crash in bpo-42972:
https://bugs.python.org/issue42972#msg394520
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GH-26399 is failing with an access violation on Windows. It's failing in one of the flaky tests. I wonder if the segfault is related to flaky tests somehow...

https://dev.azure.com/Python/cpython/_build/results?buildId=81570&view=logs&j=c83831cd-3752-5cc7-2f01-8276919eb334

test_pha_optional (test.test_ssl.TestPostHandshakeAuth) ... ok
test_pha_optional_nocert (test.test_ssl.TestPostHandshakeAuth) ... ok
test_pha_required (test.test_ssl.TestPostHandshakeAuth) ... ok
Windows fatal exception: access violation

Current thread 0x000009e0 (most recent call first):
  File "D:\a\1\s\lib\linecache.py", line 63 in checkcache
  File "D:\a\1\s\lib\traceback.py", line 375 in extract
  File "D:\a\1\s\lib\traceback.py", line 494 in __init__
  File "D:\a\1\s\lib\traceback.py", line 132 in format_exception
  File "D:\a\1\s\lib\test\test_ssl.py", line 262 in handle_error
  File "D:\a\1\s\lib\test\test_ssl.py", line 2530 in run
  File "D:\a\1\s\lib\threading.py", line 1006 in _bootstrap_inner
  File "D:\a\1\s\lib\threading.py", line 963 in _bootstrap

Thread 0x000003c4 (most recent call first):
  File "D:\a\1\s\lib\threading.py", line 1102 in _wait_for_tstate_lock
  File "D:\a\1\s\lib\threading.py", line 1086 in join
  File "D:\a\1\s\lib\test\test_ssl.py", line 2604 in run
  File "D:\a\1\s\lib\threading.py", line 1006 in _bootstrap_inner
  File "D:\a\1\s\lib\threading.py", line 963 in _bootstrap

Thread 0x00001700 (most recent call first):
  File "D:\a\1\s\lib\ssl.py", line 1131 in read
  File "D:\a\1\s\lib\ssl.py", line 1256 in recv
  File "D:\a\1\s\lib\test\test_ssl.py", line 4471 in test_pha_required_nocert
  File "D:\a\1\s\lib\unittest\case.py", line 549 in _callTestMethod
  File "D:\a\1\s\lib\unittest\case.py", line 592 in run
  File "D:\a\1\s\lib\unittest\case.py", line 652 in __call__
  File "D:\a\1\s\lib\unittest\suite.py", line 122 in run
  File "D:\a\1\s\lib\unittest\suite.py", line 84 in __call__
  File "D:\a\1\s\lib\unittest\suite.py", line 122 in run
  File "D:\a\1\s\lib\unittest\suite.py", line 84 in __call__
  File "D:\a\1\s\lib\unittest\runner.py", line 176 in run
  File "D:\a\1\s\lib\test\support\__init__.py", line 959 in _run_suite
  File "D:\a\1\s\lib\test\support\__init__.py", line 1082 in run_unittest
  File "D:\a\1\s\lib\test\test_ssl.py", line 5007 in test_main
  File "D:\a\1\s\lib\test\libregrtest\runtest.py", line 246 in _runtest_inner2
  File "D:\a\1\s\lib\test\libregrtest\runtest.py", line 282 in _runtest_inner
  File "D:\a\1\s\lib\test\libregrtest\runtest.py", line 154 in _runtest
  File "D:\a\1\s\lib\test\__main__.py", line 2 in <module>
  File "D:\a\1\s\lib\runpy.py", line 86 in _run_code
  File "D:\a\1\s\lib\runpy.py", line 196 in _run_module_as_main
##[error]Cmd.exe exited with code '-1073741819'.
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Erlend E. Aasland added:
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Hm, I'm unable to reproduce it w/addr sanitiser on macOS (FWIW).

$ ./python.exe -m test test_ssl -F -u all -m test_pha_required_nocert

Passing 1000 successful runs now. I'll see if I can get a Win dev env set up later.
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I may be related to commit dcb8786a9848516e823e090bb36079678913d8d3 "bpo-42972: Fully implement GC protocol for ssl heap types (GH-26370)".