I suspect the worker is reporting an error and it's being propagated to the window's onerror handler, sometimes before onload and sometimes after. ``` { "status": "ERROR", "group": "default", "message": "unexpected token: ':'", "stack": "http://web-platform.test:8000/workers/constructors/Worker/does-not-exist.js:1:8", "subtest": null, "test": "/workers/constructors/Worker/Worker-constructor.html", "line": 142571, "action": "test_result", "expected": "OK" } ```