In the latest web-platform-tests import CL (https://codereview.chromium.org/2648173006), I observed the following output from this test in some try jobs:

Start of test output:

CONSOLE ERROR: line 15: Uncaught (in promise) Error
CONSOLE ERROR: line 24: Uncaught (in promise) Error
CONSOLE ERROR: line 541: Uncaught (in promise) Error
CONSOLE ERROR: line 560: Uncaught (in promise) Error
CONSOLE ERROR: line 577: Uncaught (in promise) Error
CONSOLE ERROR: line 600: Uncaught (in promise) Error
CONSOLE ERROR: line 625: Uncaught (in promise) Error
CONSOLE ERROR: line 667: Uncaught (in promise) Error
CONSOLE ERROR: line 2684: Uncaught TypeError: Cannot read property 'message' of undefined
CONSOLE ERROR: line 700: Uncaught (in promise) undefined
CONSOLE ERROR: line 2684: Uncaught TypeError: Cannot read property 'message' of undefined
CONSOLE ERROR: line 767: Uncaught (in promise) undefined
CONSOLE ERROR: line 62: Uncaught (in promise) Error
CONSOLE ERROR: line 120: Uncaught (in promise) Error
CONSOLE ERROR: line 131: Uncaught (in promise) Error
CONSOLE ERROR: line 157: Uncaught (in promise) Error
CONSOLE ERROR: line 35: Uncaught (in promise) Error
CONSOLE ERROR: line 2684: Uncaught TypeError: Cannot read property 'message' of undefined
CONSOLE ERROR: line 703: Uncaught (in promise) undefined
CONSOLE ERROR: line 2684: Uncaught TypeError: Cannot read property 'message' of undefined
CONSOLE ERROR: line 735: Uncaught (in promise) undefined
CONSOLE ERROR: line 48: Uncaught (in promise) Error
CONSOLE ERROR: line 78: Uncaught (in promise) Error
CONSOLE ERROR: line 98: Uncaught (in promise) Error
CONSOLE ERROR: line 142: Uncaught (in promise) Error
CONSOLE ERROR: line 15: Uncaught (in promise) Error
CONSOLE ERROR: line 24: Uncaught (in promise) Error
CONSOLE ERROR: line 541: Uncaught (in promise) Error
CONSOLE ERROR: line 560: Uncaught (in promise) Error
CONSOLE ERROR: line 577: Uncaught (in promise) Error
CONSOLE ERROR: line 600: Uncaught (in promise) Error
CONSOLE ERROR: line 625: Uncaught (in promise) Error
CONSOLE ERROR: line 667: Uncaught (in promise) Error
CONSOLE ERROR: line 62: Uncaught (in promise) Error
CONSOLE ERROR: line 120: Uncaught (in promise) Error
CONSOLE ERROR: line 131: Uncaught (in promise) Error
CONSOLE ERROR: line 157: Uncaught (in promise) Error
CONSOLE ERROR: line 35: Uncaught (in promise) Error
CONSOLE ERROR: line 48: Uncaught (in promise) Error
CONSOLE ERROR: line 78: Uncaught (in promise) Error
CONSOLE ERROR: line 98: Uncaught (in promise) Error
CONSOLE ERROR: line 142: Uncaught (in promise) Error
This is a testharness.js-based test.
Harness Error. harness_status.status = 1 , harness_status.message = 
...

The order of these errors was different on different runs.

I think that the reason for these error messages may be that testharness.js was changed.