test_pathlib contains, in PosixPathTest.test_expanduser, a check that expanduser on a nonexistent user will raise RuntimeError. Leaving aside the question why that's a RuntimeError (which is probably too late to fix anyway), the test performs this check by assuming 'fakeuser' is a nonexistent user. This test will fail when such a user does exist. (The test already uses the pwd module for other reasons, so it certainly could check that first.)