We (Chrome OS toolchain team) run several builds everyday as tryjobs.
Over the past week, we've seen about 20 of these fail during the Sync step.

The failure is always prefixed by:

WARNING: return code: 1; command: repo manifest -o - -r
Traceback (most recent call last):
   <stack trace omitted>
error.GitError: rev-parse: [Errno 2] No such file or directory: '/b/cbuild/internal_master/src/aosp/external/dbus-binding-generator'

cmd=['repo', 'manifest', '-o', '-', '-r'], cwd=/b/cbuild/internal_master, extra env={'PAGER': 'cat'}
BUILDROOT: /b/cbuild/internal_master
TRACKING BRANCH: master
NEXT MANIFEST: official.xml
01:46:13: INFO: RunCommand: repo manifest in /b/cbuild/internal_master
01:46:13: INFO: RunCommand: repo selfupdate in /b/cbuild/internal_master
info: A new version of repo is available


object 69034721e607ecbe8dad736c8ce07d91efdc8353
type commit
tag v1.12.37-cr1
tagger Mike Frysinger <vapier@chromium.org> 1476811349 -0400

Chromium-specific release

gpg: Signature made Tue 18 Oct 2016 10:22:29 AM PDT
gpg:                using RSA key DA03FD3916B500A8
gpg: Can't check signature: public key not found


warning: Skipped upgrade to unverified version



This is followed by later errors about --cache-dir:

> main.py: error: no such option: --cache-dir


Recent examples of this failure:

- https://uberchromegw.corp.google.com/i/chromiumos.tryserver/builders/release/builds/8344
- https://uberchromegw.corp.google.com/i/chromiumos.tryserver/builders/gcc_toolchain/builds/178