https://tbpl.mozilla.org/php/getParsedLog.php?id=29199817&tree=Cedar (ignore the crash, that's a separate issue). 10:44:14 INFO - 35514 INFO TEST-START | /tests/content/base/test/test_copyimage.html 10:44:14 INFO - 35515 ERROR TEST-UNEXPECTED-FAIL | /tests/content/base/test/test_copyimage.html | uncaught exception - Error: Permission denied to access property 'toString' at :0 10:44:14 INFO - JavaScript error: , line 0: Permission denied to access property 'toString' 10:44:14 INFO - 35516 ERROR TEST-UNEXPECTED-FAIL | /tests/content/base/test/test_copyimage.html | uncaught exception - Error: Permission denied to access property 'message' at :0 10:44:14 INFO - JavaScript error: , line 0: Permission denied to access property 'message' 10:44:14 INFO - 35517 ERROR TEST-UNEXPECTED-FAIL | /tests/content/base/test/test_copyimage.html | uncaught exception - uncaught exception: unknown (can't convert to string) at :0 10:44:14 INFO - JavaScript error: , line 0: uncaught exception: unknown (can't convert to string) 10:44:14 INFO - 35518 ERROR TEST-UNEXPECTED-FAIL | /tests/content/base/test/test_copyimage.html | [SimpleTest.finish()] this test already called finish! 10:44:14 INFO - 35519 ERROR TEST-UNEXPECTED-FAIL | /tests/content/base/test/test_copyimage.html | called finish() multiple times 10:44:14 INFO - 35520 ERROR TEST-UNEXPECTED-FAIL | /tests/content/base/test/test_copyimage.html | [SimpleTest.finish()] this test already called finish! 10:44:14 INFO - 35521 ERROR TEST-UNEXPECTED-FAIL | /tests/content/base/test/test_copyimage.html | called finish() multiple times 10:44:14 INFO - 35522 INFO TEST-END | /tests/content/base/test/test_copyimage.html | finished in 173ms I've also noticed this when running locally with mach: ./mach mochitest-b2g-desktop

Note, there's a good chance we shouldn't even be running this on b2g desktop, if that's the case we'll disable and move on.

What makes you blame test_classList.html?

Perhaps I was jumping to conclusions there. test_copyimage.html doesn't use toString AFAICS, but the previous test test_classList.html does. However, test_classList.html doesn't use "message" AFAICS. I haven't found a single previous test that uses both toString and message. I suggest running individual tests, or smaller subsets to confirm which test is responsible. There are 2 extra SimpleTest.finish() calls and I don't know how test_copyimage.html could do that.