Failed here: https://evergreen.mongodb.com/task/mongo_python_driver_42dev_tests_windows_vs2015_python_version_27plus__auth_ssl~auth_ssl_windows_vs2015_python_version~3.6_test_latest_sharded_cluster_ecc852c3221447e79a3105aa949869d26a708d16_18_12_06_23_28_58
I think the issue is that in this particularly unlucky test suite all 20 operations ran against the same mongos as the original transaction. With 2 mongoses, 20 attempts yields a 1 in 1048576 chance of a false positive so we got very unlucky here. Increasing to 50 attempts yields a 1 in 1125899906842624 false positive chance. We should increase the number of operations to ensure this test doesn't flake in the future.