In e.g. https://ci.chromium.org/buildbot/chromium.infra.cron/wpt-exporter/83173 we had errors that looked like the following while trying to create a pull request for https://chromium-review.googlesource.com/c/chromium/src/+/1072471:

Traceback (most recent call last):
  File "/b/rr/tmpbIgCFY/w/src/third_party/blink/tools/wpt_export.py", line 28, in <module>
    main()
  File "/b/rr/tmpbIgCFY/w/src/third_party/blink/tools/wpt_export.py", line 20, in main
    success = exporter.main()
  File "/b/rr/tmpbIgCFY/w/src/third_party/blink/tools/blinkpy/w3c/test_exporter.py", line 75, in main
    self.process_gerrit_cls(open_gerrit_cls)
  File "/b/rr/tmpbIgCFY/w/src/third_party/blink/tools/blinkpy/w3c/test_exporter.py", line 106, in process_gerrit_cls
    self.process_gerrit_cl(cl)
  File "/b/rr/tmpbIgCFY/w/src/third_party/blink/tools/blinkpy/w3c/test_exporter.py", line 131, in process_gerrit_cl
    self.create_or_update_pr_from_inflight_cl(cl)
  File "/b/rr/tmpbIgCFY/w/src/third_party/blink/tools/blinkpy/w3c/test_exporter.py", line 274, in create_or_update_pr_from_inflight_cl
    commit, provisional=True, pr_footer=footer, pr_branch_name=branch_name)
  File "/b/rr/tmpbIgCFY/w/src/third_party/blink/tools/blinkpy/w3c/test_exporter.py", line 333, in create_or_update_pr_from_commit
    self.local_wpt.create_branch_with_patch(pr_branch_name, message, patch, author, force_push=updating)
  File "/b/rr/tmpbIgCFY/w/src/third_party/blink/tools/blinkpy/w3c/local_wpt.py", line 103, in create_branch_with_patch
    self.run(['git', 'push', 'origin', branch_name])
  File "/b/rr/tmpbIgCFY/w/src/third_party/blink/tools/blinkpy/w3c/local_wpt.py", line 50, in run
    return self.host.executive.run_command(command, cwd=self.path, **kwargs)
  File "/b/rr/tmpbIgCFY/w/src/third_party/blink/tools/blinkpy/common/system/executive.py", line 337, in run_command
    (error_handler or self.default_error_handler)(script_error)
  File "/b/rr/tmpbIgCFY/w/src/third_party/blink/tools/blinkpy/common/system/executive.py", line 244, in default_error_handler
    raise error
blinkpy.common.system.executive.ScriptError: Failed to run "['git', 'push', 'origin', 'chromium-export-cl-1072471']" exit_code: 1 cwd: /tmp/wpt
output: Last 500 characters of output:
m-tests.git
 ! [rejected]              chromium-export-cl-1072471 -> chromium-export-cl-1072471 (non-fast-forward)
error: failed to push some refs to 'https://fd33bd6a41f84861545b1a1cb173d1a380b11c9e@github.com/w3c/web-platform-tests.git'

The exporter is now failing a bit earlier -- e.g. https://ci.chromium.org/buildbot/chromium.infra.cron/wpt-exporter/83197

2018-05-28 05:22:16,519 - Searching for exportable in-flight CLs.
2018-05-28 05:22:20,746 - Found Gerrit in-flight CL: "webrtc wpt: remove test_state_change_event" https://chromium-review.googlesource.com/1074696
Traceback (most recent call last):
  File "/b/rr/tmpexvMds/w/src/third_party/blink/tools/wpt_export.py", line 28, in <module>
    main()
  File "/b/rr/tmpexvMds/w/src/third_party/blink/tools/wpt_export.py", line 20, in main
    success = exporter.main()
  File "/b/rr/tmpexvMds/w/src/third_party/blink/tools/blinkpy/w3c/test_exporter.py", line 75, in main
    self.process_gerrit_cls(open_gerrit_cls)
  File "/b/rr/tmpexvMds/w/src/third_party/blink/tools/blinkpy/w3c/test_exporter.py", line 106, in process_gerrit_cls
    self.process_gerrit_cl(cl)
  File "/b/rr/tmpexvMds/w/src/third_party/blink/tools/blinkpy/w3c/test_exporter.py", line 115, in process_gerrit_cl
    pull_request = self.wpt_github.pr_with_change_id(cl.change_id)
  File "/b/rr/tmpexvMds/w/src/third_party/blink/tools/blinkpy/w3c/wpt_github.py", line 331, in pr_with_change_id
    for pull_request in self.all_pull_requests():
  File "/b/rr/tmpexvMds/w/src/third_party/blink/tools/blinkpy/common/memoized.py", line 45, in __call__
    result = self._function(*args)
  File "/b/rr/tmpexvMds/w/src/third_party/blink/tools/blinkpy/w3c/wpt_github.py", line 216, in all_pull_requests
    response = self.request(path, method='GET')
  File "/b/rr/tmpexvMds/w/src/third_party/blink/tools/blinkpy/w3c/wpt_github.py", line 70, in request
    headers=headers
  File "/b/rr/tmpexvMds/w/src/third_party/blink/tools/blinkpy/common/net/web.py", line 49, in request
    return opener.open(request)
  File "/b/cipd_path_tools/lib/python2.7/urllib2.py", line 435, in open
    response = meth(req, response)
  File "/b/cipd_path_tools/lib/python2.7/urllib2.py", line 548, in http_response
    'http', request, response, code, msg, hdrs)
  File "/b/cipd_path_tools/lib/python2.7/urllib2.py", line 473, in error
    return self._call_chain(*args)
  File "/b/cipd_path_tools/lib/python2.7/urllib2.py", line 407, in _call_chain
    result = func(*args)
  File "/b/cipd_path_tools/lib/python2.7/urllib2.py", line 556, in http_error_default
    raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)
urllib2.HTTPError: HTTP Error 422: Unprocessable Entity

According to https://developer.github.com/v3/ HTTP 422 is sent when one or more invalid fields are sent to GitHub's API endpoint.