Hi, I found that if to call run_until_complete() in the task while the event loop will be running, a coroutine sent to run_until_complete() will be performed despite the exception raised from run_until_complete().
It seems to me, it would be better to cancel such a coroutine or just do nothing with it.

Agree, this is a bug.

New changeset 867d8333ce6a7f74191ad464acc609d4a04e4acb by Andrew Svetlov in branch '3.8':
[3.8] bpo-39191: Don't spawn a task before failing (GH-17796) (GH-17820)
https://github.com/python/cpython/commit/867d8333ce6a7f74191ad464acc609d4a04e4acb

New changeset 9c145e19fba53369da5cd23a1e71de489836f827 by Andrew Svetlov in branch '3.7':
[3.7] bpo-39191: Don't spawn a task before failing (GH-17796) (#17821)
https://github.com/python/cpython/commit/9c145e19fba53369da5cd23a1e71de489836f827

This commit seems to generate some warnings in the nested run_until_complete test. The check for running loop is now done at run_until_complete itself before going through the below code in it to create a future and raised a RuntimeError by run_forever as the test used to do. Not sure why CI doesn't have these warnings.

PYTHONWARNINGS=always ./python.exe -m test test_asyncio -m test_run_until_complete_nesting
0:00:00 load avg: 2.16 Run tests sequentially
0:00:00 load avg: 2.16 [1/1] test_asyncio
/Users/kasingar/stuff/python/cpython/Lib/traceback.py:220: RuntimeWarning: coroutine 'EventLoopTestsMixin.test_run_until_complete_nesting.<locals>.coro1' was never awaited
  tb.tb_frame.clear()
RuntimeWarning: Enable tracemalloc to get the object allocation traceback
/Users/kasingar/stuff/python/cpython/Lib/traceback.py:220: RuntimeWarning: coroutine 'EventLoopTestsMixin.test_run_until_complete_nesting.<locals>.coro1' was never awaited
  tb.tb_frame.clear()
RuntimeWarning: Enable tracemalloc to get the object allocation traceback
/Users/kasingar/stuff/python/cpython/Lib/traceback.py:220: RuntimeWarning: coroutine 'EventLoopTestsMixin.test_run_until_complete_nesting.<locals>.coro1' was never awaited
  tb.tb_frame.clear()
RuntimeWarning: Enable tracemalloc to get the object allocation traceback

== Tests result: SUCCESS ==

1 test OK.

Total duration: 164 ms
Tests result: SUCCESS

Ooh, you are right.
The warning is printed only, test runner thinks that the test is ok.
I'll prepare a quick fix.

Thanks for the report!

New changeset 10ac0cded26d91c3468e5e5a87cecad7fc0bcebd by Miss Islington (bot) (Andrew Svetlov) in branch 'master':
bpo-39191: Fix RuntimeWarning in asyncio test (GH-17863)
https://github.com/python/cpython/commit/10ac0cded26d91c3468e5e5a87cecad7fc0bcebd

New changeset 4112a3da2e01ecc19e9f54b8ac7b383b6f5e85c8 by Andrew Svetlov in branch '3.8':
[3.8] bpo-39191: Fix RuntimeWarning in asyncio test (GH-17863) (#17894)
https://github.com/python/cpython/commit/4112a3da2e01ecc19e9f54b8ac7b383b6f5e85c8