This happened with /html/dom/idlharness.https.html?exclude=(Document|Window|HTML.*) but I strongly suspect it's not about that specific test.
▶ Unexpected subtest result in /html/dom/idlharness.https.html?exclude=(Document|Window|HTML.*):
│ FAIL [expected PASS] idl_test setup
└ → promise_test: Unhandled rejection with value: object "TypeError: Network error occurred"