Oh, I'm happy! My regrtest change (bpo-37069) to detect unraisable exception detected its first issue :-)

AMD64 Ubuntu Shared 3.x:
https://buildbot.python.org/all/#/builders/141/builds/2029

test_thousand (test.test_multiprocessing_forkserver.WithProcessesTestBarrier) ... Warning -- Unraisable exception
Exception ignored in: <Finalize object, dead>
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/multiprocessing/util.py", line 189, in __call__
    res = self._callback(*self._args, **self._kwargs)
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/multiprocessing/synchronize.py", line 87, in _cleanup
    sem_unlink(name)
FileNotFoundError: [Errno 2] No such file or directory
Warning -- Unraisable exception
Exception ignored in: <Finalize object, dead>
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/multiprocessing/util.py", line 189, in __call__
    res = self._callback(*self._args, **self._kwargs)
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/multiprocessing/synchronize.py", line 87, in _cleanup
    sem_unlink(name)
FileNotFoundError: [Errno 2] No such file or directory
Warning -- Unraisable exception
Exception ignored in: <Finalize object, dead>
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/multiprocessing/util.py", line 189, in __call__
    res = self._callback(*self._args, **self._kwargs)
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/multiprocessing/synchronize.py", line 87, in _cleanup
    sem_unlink(name)
FileNotFoundError: [Errno 2] No such file or directory
Warning -- Unraisable exception
Exception ignored in: <Finalize object, dead>
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/multiprocessing/util.py", line 189, in __call__
    res = self._callback(*self._args, **self._kwargs)
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/multiprocessing/synchronize.py", line 87, in _cleanup
    sem_unlink(name)
FileNotFoundError: [Errno 2] No such file or directory
Warning -- Unraisable exception
Exception ignored in: <Finalize object, dead>
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/multiprocessing/util.py", line 189, in __call__
    res = self._callback(*self._args, **self._kwargs)
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/multiprocessing/synchronize.py", line 87, in _cleanup
    sem_unlink(name)
FileNotFoundError: [Errno 2] No such file or directory
ok

--

I'm unable to reproduce the issue, it looks like a race condition.

I ran the following commands 4 times in parallel and I stressed my laptop:

./python -m test --fail-env-changed test_multiprocessing_forkserver -m test_thousand -v -F