Example failures:

https://luci-milo.appspot.com/p/chromium/builders/luci.chromium.try/linux_layout_tests_slimming_paint_v2/14170
https://luci-milo.appspot.com/p/chromium/builders/luci.chromium.try/cast_shell_android/44009
https://luci-milo.appspot.com/p/chromium/builders/luci.chromium.try/ios-simulator/57063

Traceback (most recent call last):
  File "/b/swarming/w/ir/kitchen-checkout/depot_tools/gclient.py", line 3029, in <module>
    sys.exit(main(sys.argv[1:]))
  File "/b/swarming/w/ir/kitchen-checkout/depot_tools/gclient.py", line 3015, in main
    return dispatcher.execute(OptionParser(), argv)
  File "/b/swarming/w/ir/kitchen-checkout/depot_tools/subcommand.py", line 252, in execute
    return command(parser, args[1:])
  File "/b/swarming/w/ir/kitchen-checkout/depot_tools/gclient.py", line 2713, in CMDrunhooks
    return client.RunOnDeps('runhooks', args)
  File "/b/swarming/w/ir/kitchen-checkout/depot_tools/gclient.py", line 1602, in RunOnDeps
    self.RunHooksRecursively(self._options, pm)
  File "/b/swarming/w/ir/kitchen-checkout/depot_tools/gclient.py", line 1022, in RunHooksRecursively
    hook.run(self.root.root_dir)
  File "/b/swarming/w/ir/kitchen-checkout/depot_tools/gclient.py", line 233, in run
    cmd, cwd=cwd, always=self._verbose)
  File "/b/swarming/w/ir/kitchen-checkout/depot_tools/gclient_utils.py", line 314, in CheckCallAndFilterAndHeader
    return CheckCallAndFilter(args, **kwargs)
  File "/b/swarming/w/ir/kitchen-checkout/depot_tools/gclient_utils.py", line 525, in CheckCallAndFilter
    **kwargs)
  File "/b/swarming/w/ir/kitchen-checkout/depot_tools/subprocess2.py", line 246, in __init__
    super(Popen, self).__init__(args, **kwargs)
  File "/b/swarming/w/ir/cipd_bin_packages/lib/python2.7/subprocess.py", line 390, in __init__
    errread, errwrite)
  File "/b/swarming/w/ir/cipd_bin_packages/lib/python2.7/subprocess.py", line 1024, in _execute_child
    child_exception = pickle.loads(data)
  File "/b/swarming/w/ir/cipd_bin_packages/lib/python2.7/pickle.py", line 1388, in loads
    return Unpickler(file).load()
  File "/b/swarming/w/ir/cipd_bin_packages/lib/python2.7/pickle.py", line 864, in load
    dispatch[key](self)
  File "/b/swarming/w/ir/cipd_bin_packages/lib/python2.7/pickle.py", line 977, in load_string
    self.append(rep.decode("string-escape"))
LookupError: unknown encoding: string-escape

https://luci-milo.appspot.com/p/chromium/builders/luci.chromium.try/android_compile_dbg/52121
https://luci-milo.appspot.com/p/chromium/builders/luci.chromium.try/android_arm64_dbg_recipe/79683
https://luci-milo.appspot.com/p/chromium/builders/luci.chromium.try/android-kitkat-arm-rel/42771

Copying gs://chromium-ar-test-apks/arcore/f8004b83fc757d7724abdd492ac74cd7a9ab8e62...
/ [0 files][    0.0 B/ 11.0 MiB]                                                
AccessDeniedException: 403 
Downloading 1 files took 4.741311 second(s)
Traceback (most recent call last):
  File "src/third_party/arcore-android-sdk/test-apks/update.py", line 18, in <module>
    sys.exit(main())
  File "src/third_party/arcore-android-sdk/test-apks/update.py", line 14, in main
    '-d', ARCORE_DIR])
  File "/b/swarming/w/ir/cipd_bin_packages/lib/python2.7/subprocess.py", line 186, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['download_from_google_storage', '--bucket', 'chromium-ar-test-apks/arcore', '-d', '/b/swarming/w/ir/cache/builder/src/third_party/arcore-android-sdk/test-apks/arcore']' returned non-zero exit status 1
Error: Command '/b/swarming/w/ir/cache/vpython/998566/bin/python src/third_party/arcore-android-sdk/test-apks/update.py' returned non-zero exit status 1 in /b/swarming/w/ir/cache/builder

The first crash (unknown string-escape) is usually considered a bad state of the bot, and a reboot sometimes helps.

The second error seems to be access problems - I'll look into that shortly.

+Foundation-Troopers for ideas - I'm trying to figure out the 403 error on a public bucket.

gs://chromium-ar-test-apks is public - "Storage Legacy Bucket Reader" has allUsers:

https://pantheon.corp.google.com/storage/browser?project=chromium-archive&prefix=chromium-ar-test-apks

In particular, the .apk is publicly accessible: 
https://pantheon.corp.google.com/storage/browser/chromium-ar-test-apks?project=chromium-archive&prefix=arcore%2Ff8004b83fc757d7724abdd492ac74cd7a9ab8e62

I'm not sure how it can fail with 403... GS flakiness?

Detected 3 new flakes for test/step "gclient (with patch)". To see the actual flakes, please visit https://chromium-try-flakes.appspot.com/all_flake_occurrences?key=ahVzfmNocm9taXVtLXRyeS1mbGFrZXNyHwsSBUZsYWtlIhRnY2xpZW50ICh3aXRoIHBhdGNoKQw. This message was posted automatically by the chromium-try-flakes app.

Detected 3 new flakes for test/step "gclient (with patch)". To see the actual flakes, please visit https://chromium-try-flakes.appspot.com/all_flake_occurrences?key=ahVzfmNocm9taXVtLXRyeS1mbGFrZXNyHwsSBUZsYWtlIhRnY2xpZW50ICh3aXRoIHBhdGNoKQw. This message was posted automatically by the chromium-try-flakes app.

Detected 3 new flakes for test/step "gclient (with patch)". To see the actual flakes, please visit https://chromium-try-flakes.appspot.com/all_flake_occurrences?key=ahVzfmNocm9taXVtLXRyeS1mbGFrZXNyHwsSBUZsYWtlIhRnY2xpZW50ICh3aXRoIHBhdGNoKQw. This message was posted automatically by the chromium-try-flakes app.

Detected 3 new flakes for test/step "gclient (with patch)". To see the actual flakes, please visit https://chromium-try-flakes.appspot.com/all_flake_occurrences?key=ahVzfmNocm9taXVtLXRyeS1mbGFrZXNyHwsSBUZsYWtlIhRnY2xpZW50ICh3aXRoIHBhdGNoKQw. This message was posted automatically by the chromium-try-flakes app.

I don't find this issue actionable. The bug filing has stopped, although flakes continue to show up in chromium-try-flakes. They are rare and are all over the fleet with no obvious pattern. Archiving.